THIS IS NOT LEGAL ADVICE:
I recently built a featureless AR15 and like you, I found threadless barrels impossible to find. So I went the route you went and had the same thoughts about how to attach the thread protector to be compliant.
This is what it says on the NY Safe Act web site is ...
Walk into Cabela's and look at their gun selection and tell me they are "fully stocked". Call DD's Ranch and ask him if he's fully stocked. I don't think this is exaggerated at all. It's also getting worse.
Regarding the video, the guy says multiple times "This may look like a lot of guns...