I think seeing Andor was rage and lashing out at the man who set him on the path to losing his religion, as it were.

It’s partially that, but it’s also a little spark of hope in Syril after having his whole world ripped away.

Remember, he believed he was sent to Ghorman to root out outside agitators. He thought his ISB believed people like Andor were active with the Ghorman Front, and that his mission was to hunt down and expose them. Then he had the realization that was a lie the whole time. That, really, he, Syril, was the outside agitator. That he had been played by both the ISB and his own girlfriend into astro-turfing the Ghorman Front into something militant enough to enable the genocide of Ghorman. He has his entire world shattered.

Then he saw Andor in the crowd. That was a sign that his whole mission hadn’t been a front. There really were outside Rebel agitators. Not only that, but it was the very guy Syril had been so dogmatically chasing and hunting for so long. Like you said, Andor was the man that set Syril on the path that inevitably let him to Ghorman. Now it turns out (as Syril is suddenly believing) that Andor was also the person he was hunting on Ghorman this whole time. In Syril’s mind, he’s now realizing that Andor has been at the center of everything important (in Syril’s mind) he’s ever done in his life. That’s why the “who are you?” line is such a gut-punch. He’s obsessing over Andor. He built his whole ideology and world-view around the myth of Andor (and people like Andor) Syril kept in his head. Only to learn this guy doesn’t even remember him?

I think Syril’s rage and attack on Andor was Syril trying to redeem himself in some way. Like, if I can just bring in proof that this outside Rebel agitator really was here, that the Ghorman Front really always was the militant force the Empire is now making them out to be, that Syril could convince at least himself that he wasn’t so directly responsible in their genocide.