Disclaimer: this is purposefully obtuse.
Other effects in the game which explicitly state they kill you:
Shadows, succubi, massive damage, death saving throws, beholder death ray (notably not even their disintegration ray kills you), power word kill, vampires, mind flayers, night hags, drow inquisitors.
Clearly, if they intended for disintegration to kill you, they'd have said so. Since specific overrides general, and there is no general rule that disintegrated creatures are dead, I rest my case. QED.
So? That doesn't make it a playable option. Point to where it says, RAW, that you can continue to play as something you're turned into?
RAW, it is not a playable character option. Sounds to me like you prefer to abide by RAI...
I don't need to point to where RAW says that I cannot play it because nothing leads one to believe that you can't. If your character is polymorphed, its state changes but you can still play it.
Where does it say that, RAW? I'm using your own logic against you. You'll have to come up with a better response than that.
Edit: to clarify, where does it say RAW that you get to continue playing when polymorphed into a non-playable character?
OP could've just repeated "spells do what they say they do." It doesn't say you lose control of your character in the new form, all it says about the new form and how that affects the character is...well, that line(plus a few other things about the gear they were wearing and whatnot).
If it makes a character into a non playable character option, what rule is suddenly making that character playable? Nothing explicitly states that you can play as a character that is not a playable character option. You have a set list of RAW character options.
But nothing explicitly states that you stop playing as the character you were playing as if they were transformed, whether into a playable character option or not. There is no rule saying that that character is playable, but there is no rule saying you can't play as them. Again, spells do what they say they do.
You seem to not be understanding so let me break it down.
Playable - you can play as this
You have a list of playable character options in the character creation section of the phb, and additional ones provided by sourcebooks.
If it isn't a playable character option, it isn't playable. RAW. It isn't an option. It isn't playable.
RAW. Let me get that through your head.
"Rules as written"
Not RAI.
Not "rules as intended"
RAW.
Written.
No, you're not understanding. There is nothing saying, RAW, you can't play as a gnoll. The RAW is that you can't choose a gnoll as your race - but in the case of being polymorphed into one, you didn't choose that, you were changed into one. And the RAW for that is to assume that everything works the same as before, barring what is explicitly said in the spell for that transformation. And once again, nothing in the spell says you stop playing as a character who is transformed, regardless of if the transformation is "playable" or not. There is not even a "playable" tag for anything, anywhere, in RAW.