Yes, any kind of frequency lowering would be distortion in the strictest sense of the semantic because you’re adding coloration to the original content.
So if the word distortion is being observed in its strictest sense here, then I would want to rephrase it as “unpleasant musical aberration” instead.
If you use compression to lower the frequency, the whole spectrum of the range is compressed, so with everything squished in, the aberration may become more noticeable.
If you minimize the compression and copy a chunk of the higher range and transpose it to a lower range, BUT leaving the original higher range intact and amplified just as it would be like before, then you’re just adding on top and not altering what was there before. If what you’re adding on top doesn’t disturb the harmonics as badly as squishing everything does, the whole musical sound may be still acceptable, especially if you can control the volume amount of what you’re adding on top.
For purists, if you listen to music only, of course you don’t want to add anything to it. But if you’re in a mixed environment where you need that frequency lowering for better speech intelligibility, and there’s also background music going on, or if you’re watching a movie with both speech and music combined, if you don’t find the musical aberration to be unacceptable, then it’d be a good compromise to have in that mixed environment just so you can also hear the dialogs in the movies better.
That was my intention originally when I added Speech Rescue to my default program, and I didn’t have it in my music program. But then I noticed that I didn’t mind how the music sounds very much when I watched movies. And because I’m not a purist when it comes to music, and would rather be able to hear some of the highs in music (that I have lost entirely due to my severe high frequency loss) instead of hearing duller music, so I added Speech Rescue to my Music program as well and I’ve been happy with it.