I don't actually agree with the assertion, but I can see at least one coherent way to argue it. The thinking would be:

The world is currently very prosperous due to advances in technology that are themselves a result of the interplay between Enlightenment ideals and the particular cultures of Western Europe and America in the 1600-1950 era. Democracy is essentially irrelevant to this process - the same thing would have happened under any moderately sane government, and indeed most of the West was neither democratic nor liberal (in the modern sense) during most of this time period.

The recent outbreak of peace, meanwhile, is due to two factors. Major powers rarely fight because they have nuclear weapons, which makes war insanely risky even for ruling elites. Meanwhile America has become a world-dominating superpower with a vested interest in maintaining the status quo, so many small regional conflicts are suppressed by the threat of American intervention.

That gets us to "democracy/liberalism" doesn't get credit for making things better. To go from there to "democracy / liberalism makes things worse" you just have to believe that modern liberal societies are oppressive in ways that plausible alternatives wouldn't be, which is somewhat plausible if your personal values conflict with liberal thinking.

In reality I suspect that the alternative histories mostly involve autocratic governments banning innovation and fighting lots of pointless wars, which is why I don't buy the argument. But the evidence that liberal democracy is better than, say, a moderately conservative republic or a constitutional monarchy, is actually pretty weak. The problem is the nice alternatives to democracy are rare, because normally a country that starts moving away from autocracy ends up falling completely into the populism attractor instead of stopping somewhere along the way.