How many Celtic games did you watch last season? Southampton, for example, have a couple of ex-Celtic players who failed miserably in Scotland and start for them regularly.
This works both ways, though. Celtic have several players in their squad who were distinctly average at Championship level.
An even larger number of players go the other way, from Scotland to England, and do terrible. We have two of them. Mark Reynolds and Giles Coke, both highly rated at SPL sides, both look like boys amongst men at League One level, nevermind Championship.
For what its worth I did specifically say Celtic would probably do enough to stay up and I did specify looking at the squad as a bunch of names, rather than actually watching the team.