That statement looks very iffy to me. I see it as a strong claim resting on a bunch of unstated assumptions and with no evidence to support it (don't forget about the difference between "a personal investor" and "an average personal investor").
I originally had 'layman' in there too but decided the statement was too wordy and the context sufficiently clear. In any case the strength of your disagreement and in particular "no evidence to support it" suggests to me that your disagreement may extend to a question of fact about how efficient the market is.
Yes, I suspect so :-) However the core of my objection is the transformation of something like "An average retail investor playing in the US equity market will not beat S&P500 returns" into "It is never going to be worthwhile for a personal investor to attempt to beat the market". There are a great many markets in the world, not all of them are large and/or efficient, and, on top of that, the diversity of personal investors is much greater than the diversity of markets.
Not to mention that if you just leverage your investment a bit and the market in which you invest has a positive drift, then you will "beat the market" pretty trivially.
P/S/A: There are single sentences which can create life-changing amounts of difference.