Right at the end he says (possibly paraphrased) "I'm sure I've got some/lots of this wrong.

Well, yes, he has; but it still gets across the wonder of what MIDI actually achieved.

Electricity may be able to travel very quickly, but single-ended optocoupled circuits of that time changed state relatively slowly, so part of the reason for the three-octet (syn three-byte) data was to assure that one could get enough data down a low-cost wire to make the instruments responsive. We don't need no steenking latency laugh Optocouplers to help avoid voltage mismatches, earth loops and so on.

Many similar data formats of the time used fractions of an octet for data ... if you want minimal data to be efficient, that's just how it had to be. There was another benefit, though. He mentions microprocessors, but around 1981, microprocessors were still quite expensive (though dropping in price at a phenomenal rate), and the data could be encoded and decoded simply by low-cost logic chips ... no need for expensive microprocessors.

Using low cost components that would do the job properly translates into both a potential for more profit and a potential for more cost-effective products.

"Dirty little secret"? Actually, I think not. I think they acheived exactly what they'd hoped.
Today, MIDI data seems arcane and a bit clumsy. Forty-odd years ago it was clever, effective, and did bring synthesizer companies' interfaces together. Moistly, anyway.

FWIW, at about the same time, I was designing a system that used 16kbps data in a token-passing packet network system, capable of being passed along telephone pairs at distances up to 2 kilometers (about a mile and a quarter), also on a budget. Now we have systems like ADSL that pass way more data over greater distances. I think we didn't have the technology back in about 1980, or if we did it was proihibitively expensive.

Jazz relative beginner, starting at a much older age than was helpful.
AVL:MXE Linux; Windows 11
BIAB2025 Audiophile, a bunch of other software.
Kawai MP6, Ui24R, Focusrite Saffire Pro40 and Scarletts
.