Alembic point cloud import support was supposed to be handled by https://archive.blender.org/developer/D11592 which I recently updated, and will be the subject of a pull request in the very short…
You're right, and there is no different code path for files sequences, so this may break; I did not think of that case. I do have some Alembic file sequences, so I can use those to correct the logic.
I did try that originally but did not notice any difference with or without. Although, we could argue that not having an explicit check on our side is betting on the implementation of memcmp. I would venture that any serious implementation of memcmp would return 0 if the pointers are the same.
Just to note, memcmp gives about 1-2 fps speedup in the file from the bug report (~150k faces).
This causes a crash in one of my test file as draw_subdiv_invalidate_evaluator_for_orco is not checking the validity of the Subdiv pointer.