To get that crop reach advantage, you need a GREAT lens. A lens like the 100-400 or the Tamron 150-600 is not sharp enough.
I'll challenge anyone to go out with a 1DX or 5DII or III and get a moon shot like this one with a 100-400L.
As I said...not even close.
Oh, I missed the bit where you didn't mention the TC, that was naughty of you. There are loads more FF images out there with 5D MkII/III's with TC's that are every bit as good as yours, I was just looking for 100-400's.
Why? That shows that the bare lens was sharp enough to be dramatically out resolving the sensor, which itself had pixels 1.6x smaller than those in the 1DX. Oh, and this was a 2x TC and this shot was slightly better than the same shot taken with just a 1.4x. That's like a 72MP 1.6 crop sensor or a 184MP full frame sensor. And that's on a 16 year old zoom lens.