It's hard to disagree with them, we do need that bedrock of more linear progressive growth. But your teachers may not have the same vision for you as you do for yourself.
It's too convenient I think to say 'Once I reach x milestone I'll be able to learn everything.' Probably yes but there may be ceilings in place on things like the oscillations in Chopin op 25 no 6 or Ravel Ondine, the left hand tremolo in Don Juan at the beginning, the giant chords in Stravinsky Russian danse, the sheer density of the double notes and chords or unique double note challenges like in Liszt s140 paganini etudes, both hands having to play crazy jumps in Scriabin sonata no 5 or Schumann Fantasie, I think need an age or maturity of muscle memory ie it helps to set a longer runway.
Your brain accomplishes a lot even over really long periods of time with zero interaction with the material. A lot of things I spent 3 hours on 10+ years ago gives such a huge leg up on starting on something now. If you plant the green shoots with sightreading and even working up to say 60-70% tempo, it will actually survive a lot and contribute to learning something 'officially' later on.
Just the exposure to the new material alone triggers a lot of problem-solving at the subconscious level. You have to think of it as very valuable, for your brain to experience something that revises its understanding of limit or range or what you might encounter. Anytime you realize 'Huh octaves or chord passages can be this dense? Chromatic third sections are always short, but this is like double or triple or quadruple the length? Repeated notes are always done in groups of 3 or 4 max before changing notes but this time it's like 30.
'The natural interval or span is the octave. Oh there's a ninth? That's super exotic, I might see those but not tenths. Oh I see some tenths, that's got to be the limit right? Oh I see an eleventh, that's super unpianistic, that has to be the limit right? Jumps can only be so difficult and usually for one hand. Oh ok both hands are jumping double octaves and changing directions at the same time.' 'Every rhythm and polyrhythm will be really easy and simple.' 'My wrist and elbow only needs to ever rotate or flare out to this point. Oh this piece requires the elbow to be way further out than that.'
It's a meaningful and exciting thing to dispense with those, technique will develop differently and toward a different ceiling if you have foreknowledge of what you'll require of it. Your view of what is 'unpianistic' will be more and more different if you keep making these exploratory efforts.
A counterargument I anticipate is that it's too overwhelming - no, it's not. All it is is setting more accurate mental boundaries of what you might encounter in the repertoire, then you can prioritize based on what pieces are most important and dear to you.
What you've done now helps a lot in breaking this 'octave span' bias, which is reinforced for the first several years of piano study through classical, baroque, early romantic repertoire since ninths and above are so rarely encountered at this stage. Imagine if you hadn't done this, a year or so later, you'll wish you had to give yourself more comfortability and experience with the bigger intervals. So I think this is exactly the right thing to do, to always have a piece or excerpts to work on toward broadening what you want your technique to be able to handle. While making sure you still learn the 'right now' repertoire your teachers give you.