A covert signal is still beneficial even if the signal is secure. The existence of the signal is valuable metadata.

For a contrived example, imagine I'm in a warzone:

- Secure = Enemies can't read my messages. Good. But they can still triangulate my position.

- Covert = Enemies don't know I exist

DSSS is sort of both security and obscurity at the same time. The very act of spreading your spectrum out via a secret key also has the effect of reducing the amplitude of your transmission, ideally below the noise floor. A receiver on the other side wouldn't see anything except noise unless they had the same key.

Secure channels can still be jammed. Undetectability is a fundamentally different goal than secrecy.

I am sure you could encrypt the warmth message somehow.

Unless your adversary is scanning for RF emissions, which is getting more and more common.