That is, at best, weakly implied, and the most charitable interpretation seems to be that Swimmer963 wanted to limit his message to things he knew were true (famous, dead composers had property X) and not speculate on things he might not know (alive composers have / don't have property X).

First of all, maybe I'm misremembering, but I thought Swimmer963 was a "she".

Secondly, I can understand if maybe it doesn't bother you that people instinctively, automatically associate "composers" with "the past", but I am entitled to be bothered by it, and to correct it when I come across it.

limit his message to things he knew were true (famous, dead composers had property X) and not speculate on things he might not know (alive composers have / don't have property X).

The error of this is not in being epistemically cautious, it is in drawing the distinction between "past composers" and "present composers", thereby privileging the hypothesis that the two groups may differ with regard to the characteristics under discussion.

Here is a way that Swimmer963 could have expressed the thought without communicating the implication that contemporary composers (some of whom read Less Wrong) are too low-status to be acknowledged:

"Some composers (Mozart being a famous historical example) have incredible capacities to do this..."