It’s not new tech, Planes only know when radar locks happen because the radar ping changes to be faster and more targetted, you can’t detect a heat seeker locking onto a motor. It’s much shorter range and easier to fool if you’re planning on it than radar, since it won’t see the heat over the horisen at the distance radar guided missiles get launched at, but as long as the plane has jet engines it can’t hide from heat seeking missiles well

Pretty much, thermals require being much closer and are only good from certain directions, so the stealth is entirely focused against radar missiles that can be shot from far out of sight and don’t need to be behind the enemy