Apologies, this was over 30 years ago!

IIRC it made extensive use of lookup tables, both building them and being driven by them.

It wasn't a complete implementation, but it proved the theory and scratched an itch.

I don't mean to put down OP but isn't it kinda straight forward since assembly is just a 1:1 correspondence with machine instructions? simply pasting the assembly program into a column then looking up each mnemonic in an association table linking each mnemonic with it's machine representation and writing the result into a new column would work. The Labels->Addresses mapping is another lookup but over the program column itself, looking up the row where the label is defined and using that as the offset from the start address of the program. An assembler is just a fancy string processor, Excel is mighty good at string processing.

What I would bet a respectable sum on is that you can't parse/process/interpret even the simplest recursive languages (e.g. arithemtic, Expr ::= Expr + Expr | NUMBER) from pure Excel. Forget expressing the algorithm, there isn't even a suitable data structure to iterate over.