It's more that intelligence sources know better than to talk to the press unless told to, so it's nearly always for propaganda purposes. My experience is that the veracity of these claims are at best a coin flip.

There is a wide variety of intelligence sources.

Marco Rubio is privy to classified material and doesn't answer to the Biden administration.

And? He has his own propaganda he's trying to push and probably wouldn't be against outright lying either if it wouldn't be publicly traced back to him.

I think the point is that the public should take what “anonymous intelligence officials” say with enough salt to put the Dead Sea to shame. They might be right, they might be wrong, and they’re probably lying.

They've been successfully telling us everything Russia is doing a week ahead of time for months now.

No, but you have no idea what game the anonymous source is playing. Maybe they're telling the truth. Maybe they're telling a lie that will help them in some manner. Maybe they're just misinformed and think they're telling the truth.