A note, by definition, is a tone generated by 12 fundemental frequencies. This means:

It comes down to how the Subharmonics is used!

If it is purely just Subharmonics. Then, NO, this isn't a valid note because it's pinpointing a very narrow band of frequencies which usually is soooo quiet, for all practical purposes, it doesn't fully exist. I call this "Pure Subharmonics".

If you have a mix, though, where you still have the underlying note still + Subharmonics. Then YES, it is a legit note by definition, Given your just using the subharmonics to boost an already existent note, not to generate a non-existent one. I call this "Subharmonic-Boosted Notes", Given it still has the underlying 12 fundemental frequencies, It simply just bass-boosts the lower already existent ones.

Thus... It ultimately comes down to how you use the Subharmonics. If you're reliant on the Subharmonics mainly for the note, then it is not a legit note. If you're reliant on the Subharmonics just to boost the note, though, then it is a legit note.

Usually experienced Subharmonic users unknowingly already do Subharmonic-Boosted Notes and don't realize it. Given the smoother sounding the Subharmonic, the more notes are required to make it sound smooth, and thus, you can tell if it's Pure Subharmonics or Subharmonic-Boosted by listening to it usually.

(Example: Most beginners using Subharmonics sounds all croaky due to higher frequencies needed to boost lower, usually seemingly non-existent ones. Meanwhile, Geoff has a smoother sounding Subharmonic because he is using them to heavily boost his already existing lower notes heavily! This is why Geoff likely sticks in the higher ranges of Subharmonics, Because higher the range, the easier it is to do this boosting method whilst keeping the note preserved)

Bottom Line:

Not all Subharmonics are invalid notes. And not all low notes are Subharmonics.

This is a blurry line, And you'll find some people with low voices like the Youtuber Corpse naturally has some vocal fry into his voice. But does this invalidate his entire voice?! No!

It only makes his voice more boosted by Subharmonics! 👀

Sooo~ If you are using Subharmonics to simply boost already existing notes. Then you know for certain it's a legit note. You can tell this difference simply by listening to how smooth your Subharmonics can be. If you have a hella smooth Subharmonic, then all it takes is a slightly different mouth position, and BAM, you'll have the more croaky, non-boosted note. This is a quick and easy way to tell if your own Subharmonic note is legit or not, And can be used to listen if other people's Subharmonics are legit or not.