It's a nested liner in the sense that the lock bar "nests" inside the titanium liner. The liner lockbar is actually a separate piece that you can remove, a plus being that if the lockbar gets too worn over years of use, that you can simply just order another lockbar from Darrel and just replace it yourself. The lockbar of the HTM Gunhammer is .9 inches thick and it's massive by liner lock standards. The linerlock of the Millie isn't confidence inspiring because it looks anorexic being so thin. However, the Millie is rated as a hard use folder by Spyderco and many people will attest to that.
Here's a blown up pic of the lockbar from my own HTM Gunny. Keep in mind the one from the custom Gunhammer, such as the ones you see pictures of in this thread, is .1 inch, so slightly thicker than that of the HTM version. Is it as strong as a framelock? Probably not, but only because on a framelock you can squeeze your hand together to wedge the lockbar even tighter. Of course, unless you're prying with a linerlock and applying torque in just the right way, you won't disengage the lock. Personally, I hate framelocks. Then again, I don't pry with knives.
EDIT: If you have ever handled a Zero Tolerance 0350 and saw how thick the liner lockbar is, well the HTM Gunny lockbar is even thicker. The custom Gunhammer lockbar is thicker still.