The article literally says:
> limiting myself to mouth-speed
Audiobooks are mouth-speed.
The article suggests this is the right slow speed, at least for the author.
Maybe you yourself want even slower, but that's not what the article is suggesting.
And if you keep reading (at whatever speed), you get to the actual point of the article:
>So I tried slowing down even more, and discovered something. I slowed to a pace that felt almost absurd, treating each sentence as though it might be a particularly important one. I gave each one maybe triple the usual time and attention, ignoring the fact that there are hundreds of pages to go.
Audiobook are mouth speed but have no pause. When reading slowly, I often want to pause a few seconds and think about what I just read before moving on to the next sentence.