And all together your comment also show that you dont see my perpective, and why the thing you point doesnt matter, and what I meant with checkpoint is that you would only need real pow consencus on this checkpoint to "harden" The chain if you want to enforce a particular order on the tx/block, but that would just be about one packet saying this block height is this block hash, and having a pow once in a while on this checkpoint instead of every block.
I invented that already in collaboration with @jl777 for Komodo in 2016. It is named dPoW (delayed proof-of-work).
CounterParty does something somewhat analogous as well.
And really it isn't a secure and sound solution, but more of a gimick. Because the local consensus still have to decide what to submit for checkpoints because the PoW system isn't validating every thing and can't resolve conflicting double-spending orderings that occurred between checkpoints.