ah, that is what you meant. Yes, you cannot roll below 1 or above 20, but your chance of rolling above average increases much faster when you roll multiple d20 than when you take the maximum of multiple d6. So you are getting better much faster, you just cannot exceed 20, but then I am not sure you ever actually need to
Yeah, very few ACs or DCs require a 21+ number to succeed.
But on the other side of things, the multiple d6s in SotDL have so little impact beyond 2 that it almost makes it pointless to bother.
The first d6 raises the average by 3.5 (of course), but the second less than another +1, then less than +0.5, and barely more than +0.25 for a fourth d6.
My preference for the multiple d20s is also that the variance decreases, which is why the other DM came up with the idea of using it for "proficiency" instead of adding a proficiency bonus.
For example, a +6 prof bonus rolls 12 or better (total) 75% of the time, while rolling 5d20 (take best) results in 16 or better 75% of the time.
In other words, a higher proficiency allows makes it so your better efforts occur more commonly. If you use RAW crit on a roll of 20, multiple d20s results in a better chance for crits than the single d20 in SotDL.
Finally, the chances of rolling 20 or better is higher with SotDL than with multiple d20s. A single d6 + d20 will roll 20+ total 22.5% of the time, as where
5d20 will result in a 20 roll 22.6% of the time.
Ultimately, there are pros and cons to both methods.