Yes, that is true. Assuming you kill the command station and the special forces guard post, and anything else large with a "warp gate - full" or "warp gate - reinforce only," then that planet won't be getting any more reinforcements, so you can leave it be.
You've already paid the AI Progress for that planet at that stage, though, so you probably want to go ahead and clear it out at least well enough to travel through it even if you don't want to capture it. And it's still a risk for joining into a CPA, too, so that's another reason I tend to clear them off.
But if there are a bunch of guys under a really hard-to-kill force field or something, I've definitely left them alone while even capturing the planet in the past, it's certainly valid to do that.