Did you check all utxos one by one? In any case this statement is always true because you can just pad the scriptsig (even if that might be nonstandard).
Nice, did you bruteforce all possible valid script serializations from 0 to 3 bytes and checked no corresponding P2SH exist in the utxo set? (But also it’s not necessary to make the point it wouldn’t freeze anyone’s coins, for the same reason.)
I’m pretty sure it was discussed around Greg’s segwit ephemeral anchors (renamed ephemeral dust along the road), as alluded to by garlonicon.