What I meant was the graphics hardware rather than the systems in general, which I could have been clearer about. Looking at the registers for the 2600's TIA and 400/800/etc. CTIA/GTIA, they seem far too similar (while simultaneously being considerably different from other graphics hardware of the time) to write off as coincidence.

You have to look beyond the registers, to the whole system; those registers are largely useless without the system RAM to make them work.

Jay Miner did both then went on to Amiga. I would imagine the similarity is the learning and experience showing.