You're right. Having written quite some Z80 code and 68k code, I need to think about this. This is decades in the past, I think outside of demo code I often used move.l in 68k so it would be a 32bit CPU to me, but seldom used double registers on the Z80 so it feels like an 8bit CPU. Perhaps that was one of the reasons the jump from the Z80 to the 68k felt so huge (perhaps it was the number and flexibility of registers). That said, "Z80 assembly language subroutines" is my most precious computer book still, the one I would take through the apocalypse.

There's not much to think about. Bitness refers to the ISA, not the implementation. It's about what registers there are and what operations on them. It is what the programmer can write.

All CPUs that execute the same programs (in their native mode) are the same bitness. This is fundamental.

If a 68040 is 32 bit then so are the 68000 and 68008.

Most of the so-called "8 bit" CPUs weren't really. They were mixed 8/16 bit. Especially something like the z80 which had a lot of 16 bit registers, could do 16 bit adds and loads and stores. But not 16 bit compares or moves in the registers (needed two 8 bit moves). But even the 6800 and 6502 had certain 16 bit operations to support the 16 bit address space. The 6809 was even more 16 bit in being able to do 16 bit moves and subtracts and compares -- other than the lack of segment registers it was very very similar to the 16 bit 8088.

One problem with the terminology is that those early microprocessor instruction sets weren't Instruction Set Architectures. There was only ever one implementation of them, so people (including their manufacturers) tended to conflate implementation details and instruction set details.