In other words, I find it hard to believe a claim that subvocalization is required across the board. At least, at an absolute level.
He's talking about seeing and hearing. According to psychology, humans have two sets of "short-term memorybuffers"; spatio-visual, and auditory. It is perfectly possible to do two tasks at once, as long as you use different "buffers". Like juggling and singing.
But to be able to comprehend text, it needs to go through the linguistic centre. It doesn't matter if it goes through the spatio-visual or auditory sensors. Subvocalization is created by the linguistic centre.
Simple words or numbers do not need to go through the linguistic centre. So it is perfectly possible to comprehend text and at the same time count if you "see"/"hear" text and "hear"/"see" counting. But you cannot comprehend two advanced texts at once.
And, my entire point was that reading is different to different people. So... it may well be that many folks can not speed read. Might even be the case that most speed readers are not as good as they believe. Still, to see the numbers in this, 600wpm with 75% retention sounds ridiculously fast and not too shabby on the retention. So... yeah?