seems like a long leap there. the road team commonly use non-sponsor equipment where they believe it is better - most notably wheels, for a long time they bought their own shimano groupsets so as not to be tied to shimano for other things.
for MTB, with only 1 rider the cost of buying bikes is pretty minimal for a team with a budget of around €50M. possibly all the manufacturers they were willing to consider already had (potentially exclusive) deals with other teams or just weren't willing to fit in with performance driven requirements from ineos.
it could be seen as a sign of just how serious they are about pidcock's mtb racing that they're not willing to just stick him on whatever bike they can get someone to sponsor them with