WayOfTheFourElements
Hero
A BotE is NOT an ape.
As for apes, have you ever tried training an ape to wear armour and use a shield effectively? When you have done that, come back and show us. These are skills (like speaking English) that an ape CANNOT learn.
Well, first, as you said, it is not an ape. Therefore it might have those proficiencies hidden behind its stat block. It the creature has the ability to assume the form of any animal imaginable, I see no reason it cannot learn to form itself to suit a set of armor.