What gets to me is that this can also be a quick test of, well, rotational inertia, fabric texture and bodily flexibility. That is some pretty fricking awesome plastic modeling there.
Ah, but he didn’t know what he was looking for at the time. But it’s a fair point that nothing seemed…wrong. And this is a guy who can tell something about the wrongness of the earth by jumping up and down slightly. So yeah, incredibly sophisticated plastic-works. But then, this Nestene* probably wasn’t relying on Earth tech for its materials, and it had a lot of stake in getting it really really right. It had to fool the Doctor, after all, not a bunch of dumb apes it was just planning on shooting anyway.
* I guess I’m assuming different individual Nestenes in Spearhead/Terror, Rose, and here. The one in “Rose,” at least, died. But wait, wasn’t their homeworld lost in the Time War? It’s been too long since I’ve seen “Terror of the Autons” - is it possible that one escaped? All I’m recalling is a special effect attacking the radio tower and then being repelled. If what we’re seeing here is a Nestene who’s dealt with the Doctor before, that might explain how it was able to be so very successful this time around.