So restraint doesn't necessarily mean tying her up, or placing her in the bag, it can also cover (for instance) locking the car door so that she couldn't get out, and driving away.
It's the next part which seems ambiguous, about preventing her liberation BY holding her inside a bag in the trunk of the parked vehicle. Unless they know that Heidi was still alive in the bag when the car was parked in Houston, I think it's to do with gathering evidence that MF was instrumental in the strangulation, before they state when and by who her liberation was prevented (by killing her). I mean left like this, if they know MF put her in the bag in the trunk, and she was obviously dead on the 19th, the defense to that would have to be no she was already dead, unless someone else did it all. I doubt they can say a corpse wasn't liberated?
It seems very strange and I agree I'm sure the wording was according to the law, while they gathered more evidence.
MOO