That's odd in a purely programming way of thinking I'd have assumed it'd be easier to port from 6502 to Z80 since Z80 has a lot of instructions that have no direct equivalent and more registers. So porting from 6502 in a lazy way would in theory be easier (while netting a terrible result most likely) than the other way.
I cut my teeth on assembly programming on a Spectrum +3. I always thought the Z80s registers and extra instructions made things so much easier.