Internal registers are 16 bit, with the accumulator (A) being provisioned as two 8 bit registers (A, B) as needed. Index X, Y, Stack, User Stack, PC, are all 16 bit registers.
The Hitachi 6309, adds to that with up to 32 bit register sizes in specific cases.
In any case, the ALU and data transfers are 8 bits and I am not sure I ever saw the 6809 referenced as a 16 bit device.
Maybe 16 bit curious, LMAO.
That said, "16bit curious" is a great term :D
It certainly can punch well above its weight class, at least when compared with 6502 z80 and some others.
I really can't call it 16 bit, because of the small address space, and the fact that the ALU is 8-bit. But you can't always go by the ALU because I believe the z80 and 8080 have four bit ALUs. And I don't think there's anyone that would call those chips four bit.
Motorola seemed to design things in a specific way that people really liked, and this pushing the limits of what is an expert design seems to be one of those because even going back to the 6800, the one index register was 16 bit.
And lastly the 68k is an exemplary design, but in the same design language is 32 bit curious.