It's also quite possible Tom - having not had the same life experiences as Will - still sided with Pressman and was appalled at his former Captain being raked over the coals by Starfleet.
I dunno. Anything's possible, but the gist of young Riker's involvement in the Pegasus mutiny boiled down to not having much idea of the goings on, & when the more experienced & better briefed officers turned on him, over the project's dangerousness & questionability, a rather clueless young Riker just saw "Mutiny" & did the dutybound thing.
As Will describes it, his choice to defend Pressman didn't seem to have anything to do with agreeing with the project per say, but more just following academy trained indoctrination. Seeing what became of Tom, in how he's not really much of an officer anymore at all, after his abandonment. Something tells me he probably isn't the same kind of guy to go to bat for a captain anymore either, just because he's a captain. Certainly, falling into Maqui sympathy would kind of support that conclusion too
So it's hard for me to think Renegade Riker is still a guy who feels loyalty to Pressman, but either way, he DID more than likely get dragged into whatever tribunal took place, to give testimony, & that happening could not have looked any better for him than it did for Will (Who had the benefit of at least being the whistleblower) Which makes Tom look more like a colluder (Which he & Will & any others who hushed it up
are)
Bottom line, it sets up a rough look for Tom, back on the Gandhi, where now everyone knows what happened, & suspects him of collusion, which is probably merited, because Riker probably continued to benefit from Pressman's recommendations well after the event, as a kind of reward for keeping silent. I tend to even think it's why Will became so full of bravado, independence, & heroism afterward too. The heroism of Nervala IV itself, for which he's probably decorated, might well have been overcompensating out of guilt for having been involved in those Pegasus deaths, and for somewhat benefiting from it in his career.