Not exactly, see
here. This equivalency was only in response to my first challenge on the balance of these items.
And for me, that is a powerful effect, not an uncommon magic item.
And invocations are really powerful effects, some of which cannot be duplicated.
First no, it's not irrelevant at all, it's still a bonus. And second no, it's not at all equivalent to mithral half-plate (which is a fairly powerful armor in its own right), as contrary to medium armor (which lots of classes can't use, btw), because it allows your full DEX bonus to AC, not limiting it to +2.
Don't compare it to a half-plate, it's a bad comparison as you are totally ignoring the basic principle of bounded accuracy, the only reasonable comparison is to a legendary armor that you certainly should not be able to procure, and even less to procure easily.
And it's still only +1, and limited to those able to use a shield, and it requires one hand, which casters need in general.
I would advise not to use them in your campaigns, they are unbalanced, especially the best one which is way too powerful in a system with bounded accuracy.