I don't see any reason a gnome Battle Smith artificer (from the May 2019 UA) couldn't ride his iron defender RAW--it's got the appropriate anatomy and is one size larger.
But how is it "Controlled"? Controlled has a specific meaning in mounted combat: A controlled mount acts on the riders turn and does not get attacks, an uncontrolled mount goes on it's own initiative and gets attacks. An intelligent mount can only be uncontrolled, with an animal intelligence mount you get the choice.
I don't believe the Iron defender has a free will though, it is always controlled by the Artificer that constructed it.
So if the Iron defender was being ridden, I'm guessing it could be in "Uncontrolled" mode, going after the Artificer, and still get it's regular attack actions--but directed by the Artificer (Still essentially controlled).
On the other hand--you might want to be in "Controlled" mode if you didn't need the Iron Defender's attack--for instance if you were going to charge with a lance, have the artificer hit, then run away.
So my questions are--is my research (and assumptions) correct as I stated above, and if so, is there any way to switch between controlled and uncontrolled mode?
If you were riding an animal, I'm guessing the "Mode" might be determined by how you control it... or perhaps it cannot act on it's own if it has a bit in it's mouth and reigns? For the Iron Defender I can't think of any differentiation that would apply except the Artificer's will.