By all accounts, Saliba's return to St. Etienne for a second loan spell fell apart due to paperwork not being completed in time. Unless Mikel Arteta was filing the paperwork himself, then it was someone else's fault, and according to reports both clubs blamed each other. So the truth is, we don't know who exactly fucked up, but lazy thinkers who are completely devoid of critical thinking need a scapegoat, and the lazy option is just blame the manager.
The 6 months that followed where Saliba wasn't registered. It's not difficult to understand why a player who wasn't supposed to be at the club wasn't registered.