I see this "bad technique" angle expressed quite often when talking about self-learning. I tend to think it is overblown a bit. I started learning piano by myself during covid. Then I went to two teachers. Neither one had anything bad to say about my technique.
Teachers of beginners understand that keeping them motivated and practicing is the primary problem. Cleaning up your technique is simply not a goal. Doubly so if you are an adult learner.
Piano is also a lot harder to have very bad technique than other instruments since it is mostly a discrete, one-to-one mapping. If you want a chord with some particular notes, a lot of the time there is only a single fingering that will do. If you push the key hard enough, you will get the correct note, and it won't be wrong if you push harder.
By contrast, in guitar, if you push too hard, your note will be off even though you fretted in the correct place. Or, for example, everybody in guitar teaches barre chords up near the nut, when that's extremely difficult and likely to injure a beginner who has neither the strength nor control to get that right, instead of teaching barre chords near the body on fewer strings. etc.
I will say that I'm also speaking from personal experience, where I was self taught for about 12 years before taking my first lessons for my piano minor in uni. A single lesson resetting some habits I'd gotten into about hand posture and fingering reasoning and it was like a 50% increase in speed in a way that meant that songs that were near the top of ability almost felt effortless. It was wild
