Chuck, based on your responses, you do not seem to be making a distinction between an initial release of software as a "centralized authority" and a checkpoint that was later announced for users of an existing cryptocurrency.
Exactly, because I cannot prove the past.
So you admit that.
I think this is the heart of the matter and source of the disagreement.
From an abstract point of view, it makes no difference if the block height is 0 or if it is X when currently being at X+T and T is huge.
One could use a client using X as a checkpoint (for whatever reason) and have no problem at all.
One could use a client using 0 as a checkpoint (for whatever reason) and have no problem at all.
One could use client using X as a checkpoint (for whatever reason) and be on a fork.
One could use client using 0 as a checkpoint (for whatever reason) and be on a fork.
Substitute X and 0 with whatever symbol you see fit. They are equivalent.
EDIT: And here you see again, you have to verify or trust the last checkpoint.
I think its irrelevant. The key difference is that
one is a "contract/agreement" where all the rules of the game
are laid out at the very beginning (i.e. genesis block), and the other
one , has participants that can change the rules as they go along.
Can you not see a difference?