re rushing

I think they're referring to chapters like ADX, where they're sororities who follow the Panhel guidelines but don't participate in formal recruitment/aren't technically panhellenic. This may also apply to KBG, because at some schools they do participate in formal recruitment but aren't a part of national panhellenic.

Wrong - ADX is not an NPC sorority, so it's okay to drop them and join another house. The rule only applies to NPC sororities; once initiated into an NPC sorority you can't join another one. This is dictated on a national level.

I think the "can" is a little vague - technically you aren't banned from going through NPC recruitment if you drop a non-NPC sorority and go through recruitment. There's nothing that states that you couldn't have been in a local or religious organization or whatever. However, the groups themselves may have a different requirement for membership, and oftentimes that's an aspect of MS, which is private. So you may be allowed to rush but not get a bid because those groups may not want someone who's been in a different group before. I believe this is the case for NPHC sororities - you can't have been initiated in another GLO, regardless of conference (NPC or NPHC) so you can't be like an AKA and a DG.