It's actually not fine. I don't understand the chemistry but using an HD disk in a DD drive will appear to work, but the data will not be written "strongly" and will "fade" after a short time, becoming unreadable.
It’s coercivity of the magnetic medium. HD disks have higher coercivity than DD which means higher write current is needed to put phase transitions down on the magnetic medium when recording. There are also some differences with track widths which complicates things further if you try to reformat from HD to DD without first degaussing.
For 3.5” disks the difference is small enough that you only tend to run into problems if you format a disk as HD in an HD drive then try to reformat and reuse it in a DD drive as DD without degaussing first (because the DD drive won’t be able to fully remove traces of the previous HD signal).
For 5.25” though the difference in coercivity is quite a lot bigger and even using freshly degaussed HD discs in a DD/SD drive will very likely lead to problems quite quickly.
I have several old synths with DD drives, and I've been using HD floppies with them for ~30 years without any problem. Late HD floppies were crap (whatever the mode), but old HD floppies formatted as DD back in the early 90s still work perfectly fine. I have several hundreds of these. Some having been used hundreds of times. Some I still use weekly.
I think the trick to that is to reformat it multiple times. a DD drive has a lower intensity magnetic field, reformating it a couple times gets rid of the high intensity preformat on the HD disk.
It's not supposed to work, but I've had a floppy like this for years and it reads fine.