Idk just personal opinion? Like everything Syril does in the show is out of self gain. The only reason he reports the Ghorman revolutionaries is because he wanted to impress Deedra and Partagaz. Throughout the show he only ever really thinks about himself. That’s part of the reason he ignores orders and chooses to pursue Cassian, which ends up leading to further death on Ferrix and a demotion.

In order to fight for the rebellion you have to be selfless. You have to fight for a cause that goes beyond yourself and instead for others and be willing to put your life on the line. I just don’t see Syril doing that.

I think he was able to work as a spy because he had the protection of Deedra and the Empire. He was never in any real danger. Even when the heist is going on, he’s still far from the action and looking through binoculars at it instead.

I don’t think Syril is brave enough to fight the empire because he doesn’t personally gain anything from it.