I was taught the same about time alignment, slanted front speakers ..., the amount of time alignment difference is relative to seated listening distance of course, and the differences are very very very small, yet it could be bunk.
Related is the issue of keeping your speaker wires the same length. The lengths we use are very small relative to signal speed, but, I still follow the rule. Makes a big difference using expensive cable.
Your point about no time alignment for live music is valid, the drums are way in the back, violins up front, the opposite of what slanted front speakers do.
below they mention a band in a stadium, much greater distances.
I just found this, scroll down some
https://phys.libretexts.org/Bookshelves/College_Physics/Book%3A_College_Physics_(OpenStax)/17%3A_Physics_of_Hearing/17.02%3A_Speed_of_Sound_Frequency_and_Wavelength#:~:text=a%20smaller%20wavelength.-,Summary,s)%E2%88%9AT273K.