It would be hard to hide the odor.
And this might sound way out there, but...
They may have telemetry on BiteMe. If his vitals are being monitored, they might have seen something go south.
Interesting that BoJo covers for him.
I think it was most likely because it was an unscheduled (and unscripted) press conference, and no chance they let Biden talk without pre-written talking points.