I “agree” that the worst place to run your engine is at an EGT of around 25C rich of peak. That’s because it’s the mixture around which your CHT will be maximum and you’ll be giving your engine the hardest (and unnecessarily hardest) beating that you can give it.
I don’t “agree” or “disagree” with EDM, other than to say that their EGT/CHT charts are crap to the extent that they falsely represent CHT and EGT as ‘tracking’ each other always. There’s some other folklore in the EDM manual but I ignore it.
I don’t “agree” or “disagree” with engine manufacturers, except to the extent that some of their publications contain crap.
But whatever the facts and the bull****, doing piston engine management properly depends on you having an all cylinder engine monitor and you knowing what it’s telling you and why, and what if anything can and should be done about it.