On the other hand, de Grey and others who are primarily working on the scientific and/or engineering challenges of singularity and transhumanist technologies were far less likely to subject themselves to epistematic mistakes of significant consequences.

This part isn't clear to me. The researcher who goes into generic anti-cancer work, instead of SENS-style anti-aging work, probably has made an epistemic mistake with moderate consequences, because of basic replaceability arguments.

But to say that MIRI's approach to AGI safety is due to a philosophical mistake, and one with significant consequences, seems like it requires much stronger knowledge. Shooting very high instead of high is riskier, but not necessarily wronger.

Thankfully there is an institution that is doing that kind of work: the Future of Life institute (not MIRI).

I think you underestimate how much MIRI agrees with FLI.

Why they do not get more play in the effective altruism community is beyond me.

SENS is the second largest part of my charity budget, and I recommend it to my friends every year (on the obvious day to do so). My speculations on why EAs don't favor them more highly mostly have to do with the difficulty of measuring progress in medical research vs. fighting illnesses, and possibly also the specter of selfishness.