Essentially, I think Mat was focusing on the theoretical side to it. Mastering is essentially stepping away from the individual elements of the song that you've spent way too long mixing and just looking at it as one track. Shoving a load of processing on the master chain is essentially mixing, as you are still in the headspace of mixing the track and you'll likely go back and edit individual parts mid master section processing.

There's not really a right or wrong answer. If you wanna call putting a limiter on the master chain mastering, you're no more wrong then the rest of us. I'm just saying I think what I wrote above was the message Mat was trying to put across.

Inb4 I completely misinterpreted Mat.