It is really an 8-bit CPU from a bus (external connections) perspective...but internally, there were 16 bit registers (sort of...) Perhaps this was why he made the distinction.

The 8-bit A and B accumulators could be addressed together as a 16-bit D accumulator. The contents of A and B could be multiplied with the product stored in D. There were also two 16-bit index registers with advanced and versatile addressing modes, X and Y, along with 16-bit user and system stack pointers, U and S.

It's the link between 8-bit and 16-bit: at least that's what Motorola marketing said, look at the cover of this book:

https://www.amazon.com/MOTOROLA-MC6809E-MICROPROCESSOR-PROGR...

I have a copy of that book near me. For a very long time, Moto would send docs out for the mere asking. Free of charge. I got mine as a high schooler. Didn't use the mail though. Instead, I visited a local Moto office and they handed me the ones I wanted and I got to talk chips with the people there for a while.

For a small town kid interested in all this stuff, that made quite an impression. Instant Moto fan.

The 6809 and the 6502 had a 'fast register file' page aka the 'zero' page on the 6502 which could be relocated anywhere in memory in the 6809. These were looked at as an external extension of the register set and lots of instructions could use the data there as 8 or 16 bit data or pointers.

Not all. The Texas Instruments TMS9900 designed as a single chip version of the TI 990 minicomputer series had three internal 16 bit registers, but 16 general purpose registers in RAM.