I think the idea that Raven Guard are 'stealthy' is a bit of a misconception. You're right, you can't sneak up on someone wearing power armour.
I see Raven Guard more as masters of guerrilla tactics, ambushes, hit and run attacks etc. more than straight up stealth.
I think you're bang on for their warfare methods, but that's not how GW are writing them and that's the problem.
Plus Corax and I believe some others have the ability to make people not notice them even in plain sight. I don't know if that's a skill that has survived 10,000 years.
Smells like an inconsistent plot device to me.