ABC not accepting SV blocks is irrelevant. The goal of the checkpoint is to limit the potential depth of a reorg to the block height of the checkpoint. The attack this is meant to preclude is SV sympathizers secretly mining an ABC chain of empty blocks. If such chain exceeds the cumulative PoW of the public ABC chain, that parallel chan could be released, invalidating all previously-considered-settled txs on what is now an orphaned ABC chain.

Of course, in order to remain a viable mechanism for forestalling such an attack, the checkpointing will need to be performed periodically, as it can only guard against reorgs preceding and including the checkpoint block height.