If their primary goal was to get you to stop, I'd assume they'd send a C&D first too because it's a lot less headache for them if that makes the problem go away.
So I wouldn't see a C&D as a guaranteed sign that they aren't willing to sue, but the threshold for "want you to stop and are willing to sue for it" is much, much, MUCH higher than the threshold for "eh, I'll send a C&D and see if this makes it go away, if not, not worth it".
1) Good problem to have. 2) If they had something, they wouldn't threaten you, they'd just act.
It's the risk/reward ratio. Unless it's worth reacting to, file 13 that shit.