To clarify things, do you believe that there are measurable IQ differences between populations or you think it's all bloody nonsense made up by malicious people?

So, first of all, if you drew up a list of highly successful people in mediaeval Europe, I don't think there would be many Jews there.

Depends on whether you see the glass as half-full or half-empty. From Murray's Human Accomplishment on that topic, pg292:

Jews make their first appearance in the annals of the arts and sciences during the centuries when the Middle East and Moorish Spain were at their cultural peak. When science historian George Sarton set out to enumerate the top scientists across the world, including East Asia, South Asia, the Arab world, and Christian Europe, from 1150 to 1300, he came up with 626 names, of whom 95 were Jews—15 percent of the total, produced by a group that at the time represented about half of 1 percent of the world’s population that was in a position to produce scientists. 5 But few of those 626 are important enough in the broader sweep of scientific history to warrant a mention in histories that are less tightly focused. Of the 10 Jews who qualified as significant figures in the inventories prior to 19C, only 2 are still familiar to the general public, Montaigne and Spinoza, and neither of them was a typical Jew of his time. Montaigne’s mother came from a wealthy Spanish/Portuguese Jewish family, but Montaigne himself was a lifelong Catholic. Spinoza was excommunicated by his Dutch Jewish community for his unorthodox views.

Five of the 8 other Jews who appear in the inventories before 1800 were also part of the philosophy inventory. They were Philo Judaeus from ancient Roman Alexandria, Solomon ibn Gabirol (Avicebron) and Maimonides from Moorish Spain, and Moses Mendelssohn and Johann Herder from 18C Germany. In all of those 26 centuries, the roster of Western significant figures includes not one Jewish artist, scientist, physician, or inventor, and just one writer (Fernando Rojas), one composer (Salamone Rossi), and one mathematician (Paul Guldin).

This sparse representation in European arts and sciences through the beginning of 19C reflects Jews’ near-total exclusion from the arts and sciences. Jews were not merely discouraged from entering universities and the professions, they were often forbidden by law from doing so....I will not try to establish a hierarchy of victimhood among Jews, women, and other minorities, but an uncomplicated point needs emphasis: Until the end of 18C throughout Europe, and well into 19C in most parts of Europe, Jews lived under a regime of legally restricted rights and socially sanctioned discrimination as severe as that borne by any population not held in chattel slavery.

Personally, I would say that a 30x overrepresentation in the 1100s-1300s is rather medieval, and pretty respectable especially considering the constraints they labored under.

And obviously those constraints did make a difference because once the constraints starting being lifted, Jews began overperforming even more to a degree so absurd that if it were a novel, you'd throw it against the wall in disgust and angrily tweet at the author to look up the phrase 'Mary Sue':

This history provides us with a nice example of what social scientists call an interrupted time series. Until nearly 1800, Jews are excluded. Then, over about 70 years, the legal exclusions are lifted and the social exclusion eases. What happens? “The suddenness with which Jews began to appear . . . is nothing short of astounding,” writes historian Raphael Patai. “It seemed as if a huge reservoir of Jewish talent, hitherto dammed up behind the wall of Talmudic learning, were suddenly released to spill over into all fields of Gentile cultural activity.” 8 During the four decades from 1830 to 1870, when the first Jews to live in emancipation (or at least to live under less rigorously enforced suppression) reach their forties, 16 Jewish significant figures appear. In the next four decades, from 1870 to 1910, when all non-Russian Jews are living in societies that offer equal legal protections if not social equality, that number jumps to 40. During the next four decades until 1950—including the years of the Third Reich and the Holocaust—the number of Jewish significant figures almost triples, to 114. I do not show the results for philosophy because the Jewish proportion becomes so high in 1900–1950, when Jews represented 6 out of the 18 significant figures in philosophy (33 percent), that it distorts the other trendlines. The results shown in the graph above are already impressive enough without philosophy, as Jewish representation rises steeply in all the inventories but music, where it had begun at a high rate even in 1800–1850...To get a sense of the density of accomplishment these numbers represent, I will focus on 1870 onward, after legal emancipation had been achieved throughout Central and Western Europe. Only from this latter period can we draw a roughly accurate sense of the magnitude and patterns of Jewish accomplishment—“roughly,” because Jews were still subject to pervasive social and educational discrimination even after 1870.

(I won't quote any more since I know we're all familiar with Jewish performance in the 1900s.)

Unfortunately, I do not possess your capability of determining the sincerity of the poster on the basis of one short comment on the 'net.

Obviously stupid ideas are obviously stupid. Sincerity doesn't help them, anyway.