I wavered on Glenn during the episode, but as of the end, yeah, I think he was sincere. Team Cockroach hadn't even begun to suspect the imposter Janet -- it would've make more sense to just let her keep causing havoc in peace, and hold off on deploying Glenn until it looked like she needed support.

Also! If Glenn knew this was Bad Janet, and they were working together, she wouldn't have had to create a "demon lie detector" that blew him up. She could've just made one that marked his Michael-related lies as "true."

On a Doylist level, it's much more narratively interesting to write "the demon bloc is getting more divided and complicated" than to do yet another round of sabotage. And of the demons we've gotten to know, I'd say Glenn had the best setup to genuinely doubt the system.