The note "Important: phones may change from one category to another when manufacturers and carriers release updates." refers to obtaining root access on a new phone.
1. regardless of whether you install a custom boot rom, the lowest-level software running on the phone might be impossible to overwrite, so the carriers still have some basic low-level access to your phone to tell android to lock you out.
2. any such feature is part of the code that is itself replaced when you install a custom rom, so would be defeated and you'd be safe (at least as long as the model you buy does not turn out to have become locked down during the manufacturing batch you end up buying).
It sounds like 1 is just not the case, thankfully.