First off, Signal hasn't said anything, this is an accusation made at a conference in Kyiv. So - who knows, they're behind, they don't have billions to support an army, who knows.
IF they have chosen to not help Ukraine where at all possible, that would be bad.
All of that said, if I was running a modern army using an encrypted chat app, I'd fucking have all that shit in-house, wtf. It's 2025. Ukraine already has a bunch of l337 h4X0rs. I'm sure they could slap something together in days and have it in the field in weeks.