Assuming OP is correct, your last sentence implies this isn't the solution being used.
Additionally, many (citation needed) Youtube videos have people talking in them; this method wouldn't help with that.
Isolating vocals in general is significantly more difficult than just relying on frequency range. Any instrument I can think of can generate notes that are squarely in the common range of a human (see: https://www.dbamfordmusic.com/frequency-range-of-instruments...)
Additionally, many (citation needed) Youtube videos have people talking in them; this method wouldn't help with that.
Isolating vocals in general is significantly more difficult than just relying on frequency range. Any instrument I can think of can generate notes that are squarely in the common range of a human (see: https://www.dbamfordmusic.com/frequency-range-of-instruments...)