He said "...all the designer basically (simplistically) needs to do..."
Is nobody getting enough fiber in their diet?
He did say that, yes.
But he went beyond saying that they could do that, to, in essence, that they DID do that. He asserts that the lens will be f/5.6 at 400mm, on the basis of nothing whatsoever. He's asserting his opinion as fact.
Now I agree that IF it turns out to be f/[email protected]
, then the complaints are groundless, and they really did just extend the range without compromising what was already there. But others think it's possible that it's going to be f/6.3 at 400, in which case the now-old 100-400 L II can do 400mm better than this lens can, and it's not an improvement. We can't tell either way at this point, so saying it WILL be one way or the other is a groundless assertion.
So, contrary to your assertion that he made no assertion, he did make assertions.
(Note: I actually usually like Optics Patent's attitude; he regularly calls people out for arguing that something is useless because they personally don't want it, for instance. But in this particular case, he's jumped to what, for all the world, looks like an unwarranted conclusion and argued on that basis (an argument that would be good IF the conclusion were warranted). People are calling him on it. He can either demonstrate he's actually got good reason to believe what he says...or continue to take the heat.)
EDIT: rephrase without misleading quote marks.