but would declaring the displacement of the G3s reveal the armour-thickness shenanigans of the RN?
No its just too short and radical design to work it out.
The rules that allow you to work out thickness require you to guess how much is protected and with an internal belt that's very hard as you don't know the area covered.
The G3 (and N&R in OTL) didn't have a sufficiently long belt to cover the USN rules, ie they would sink if the rest of the ship flooded, IJN/USN also didn't fully understand why all the turrets are forward and thought at first it was for a CV/BB hybrid in OTL.
Without close inspection (OTL not till WWII) USN/IJN will not realise what exactly they are until far later such as the 1930s or WWII.