I checked the article, and i does seem to indicate that It requires a PVC strap. So are metal locknuts approved to be used with PVC male adopters? Also is there a requirement for the EI to give a code article?
1. The article only requires the support to allow movement from thermal expansion. PVC straps are designed for this but are not required. If using a rigid/IMC strap and you make it tight fitting is does not allow for thermal movement.
2. As far as metal locknuts on a PVC adapter I am not sure. Many non metallic flex fittings and cord grips have a metal locknut installed on them in the box they come in so they are most likely allowed there.
If installed in a grounded metal enclosure I see no problem at all, the locknut will be grounded by the metal enclosure that it is tightened against. But a terminal adapter with a metal locknut in a nonmetallic enclosure will not be grounded.
I have also seen nonmetallic flex connectors and cord grips that come with a nonmetallic locknut. I have seen many that are installed and the locknut has broken and the flex connector or cord grip has pulled away from the enclosure. I would rather see the metal locknut even if it is not grounded in this case. That locknut is not very likely to become energized.
BTW they do make locknuts with provisions for attaching a grounding conductor.
3. The requirement for an EI to give a code reference for violations of the code are up to the AHJ and lawmakers. I would like to think that it is required most everywhere. Otherwise it allows the inspector to make up his own code. If you are going to have rules you need to be able to provide a reference to them to prove that the rules have been broken otherwise why have the rules?
Just saying "you violated the NEC and the rules are in there somewhere" I suppose could work, but now you either give in and do as inspector requests or you get your book out and try to prove the inspector wrong.
An inspector that can not get out a code book or local amendments to that code and find an article to show you your violation is incompetent and should not be an inspector.