There's little research to back it up.
There is no magic of 20min. It's bunk.
In his first edition of the book, daniels recomended a 20min threshold at 60min race pace. In the first edition, he basically said that 20min was optimal because that's about how long most people can stand running at 60min pace in practice. He stated that paces slower or faster weren't as beneficial.
In his 2nd edition, he thankfully revised this to include longer tempo runs at slightly lower paces.
The length of the threshold run has nothing to do with percentage of total mileage.
It has to do where you are in your progression. Some people need 15minutes at that pace, some well trained runners need to train at 35min or so at that pace to get the same training effect.
That's not even taking into consideration slightly slower or faster tempos.