This is very cool! I've sometimes wondered about doing that. An obvious question is whether this suffices to show Turing completeness. I think the answer is no

This is very cool! I've sometimes wondered about doing that. An obvious question is whether this suffices to show Turing completeness. I think the answer is no: because Excel has a finite number of cells, there are programs that this Turing machine won't have enough cells for. To get around that, you'd have to do something clever with iterative calcs (i.e. circular references) that effectively reuse the same cells behind the scenes. VBA is out of the question, of course. :)