Plus in terms of the game itself, they don't have brains. The tadpole needs to literally, physically eat those brains in order to transform them. And the physicality of the creature in question does matter, otherwise halfling mindflayers wouldn't turn out so absurdly adorable. Also, where would they even come from? If they really are from Gortasb then they'd look like steel watchers, which are not gonna be able to wear armor or anything.
It is debatable.
Chris Perkins said : "Any humanoid creature, regardless of height, can be turned into a mind flayer through the process of ceremorphosis. #wotcstaff"
Jeremy Crawford : Warforged aren't constructs. They're artificial humanoids, which means the rules treat them like humanoids. #DnD
Official sourcebook Eberron : Rising from the Last War, they are humanoid. Although they were manufactured, warforged are living humanoids.
Multiverse and gates/planar travel is a theme in BG3 and is more and more trendy for Wotc.
