I think your explanation is off.

"He is not saying that one person in the group is not a good replacement. He is saying that the entire group (more than one person in the group) is not suitable."

I think the more natural reading is that he is saying that "not any" of the group of "us" is suitable, not that the entire group is not suitable. That "us" is plural is irrelevant.

Note that he says "none of us [is/are] a good
replacement" — "replacement" is singular, not plural. So it makes little sense for "none" to be plural.

The fact that "none" can be plural might be in play if he said "none of us are good replacements," but that's not the case here.