[dev1206] The effects of Protect and Shell have been made mutually exclusive

[Atoli]

Sadly, no it wasn't mistranslated. It says the exact same thing in Japanese.
The effects of Protect and Shell will not be effective at the same time... you know the rest.

[dev1206] The effect of "Shell" is added to the magic of "Protect" by a passive trait.

That's what it says in my language, and seriously, that's the only thing making any sense at all, because otherwise, how would you be using Protect at all after R24? After all, the overwriting is from a passive trait, so Protect would ALWAYS become Shell.
I think it's just formulated badly, and what actually is meant is that the original Protect effect, which only increased physical def will be overwritten with the new effect which increases both physical and magical def.