What briefly, is 7*20? I don't feel up to delving through all his posts. I know he is very highly considered here.
It was just a generalization about how many attempts (7ish) and how long (20 minuteish) one should try to learn a passage (not necessarily master it or finish working on musicality).
If it takes you more than 7 attempts, the passage or bar or whatever is too complex too learn in one sitting; therefore, you work on a smaller portion.
There also only so much improvement you can do in one sitting. If you spend more than 20 minutes on a passages, you really aren't going to get anymore benefit during that day at least in terms of motor learning. You would be better off working on another passage. 20 minutes is just a generalization, it could actually be shorter. Overtime you should learn how long you actually need to get the maximum benefit for the day for similar passages.
It's somewhat similar to the Pomodoro technique of time management, except it's for motor learning which acknowledges the additional caveat that there is a point of diminishing returns for improvement within a day.
It's also simply to break the unnecessary OCD habits pianists have that don't actually help. Again, there's a point of limiting returns within the same day.
EDIT:
He explains the basic gist here with more depth:
https://www.pianostreet.com/smf/index.php?topic=4710.msg44538#msg44538EDIT 2:
There was also a study that confirmed the basic gist behind the method.
https://cml.music.utexas.edu/assets/pdf/DukeEtAl2.pdfResearchers evaluated pianists on their ability to learn 3-measure passage from Shostakovich's Piano Concerto No. 1, and measured differet aspects of their practice behavior and how it impacted a performance trial.
Practicing longer didn’t lead to higher rankings.
Getting in more repetitions had no impact on their ranking either.
The number of times they played it correctly in practice also had no bearing on their ranking.
What did matter was:
How many times they played it incorrectly. The more times they played it incorrectly, the worse their ranking tended to be.
The percentage of correct practice trials did seem to matter. The greater the proportion of correct trials in their practice session, the higher their ranking tended to be.