OK. I've managed to get a method working again for removing the DRM from Kindle purchases (something I haven't needed or wanted to do for quite some time) and I can confirm that Kindle Magazines are not compatible with KindleImport and current versions of Sigil. I subscribed to LightSpeed which has a 28-day trial period (but I got a .mobi file and not a .pobi one for what that's worth). They do indeed import as empty epubs with tons of unmanifested files.
I have not yet tried to see if it will work with older versions of Sigil yet. But if it DID somehow accidentally work with those older versions of Sigil, I just might be able to purposely make it work again with the newer version. No promises, though.
Stay tuned!