[humanizersequel]

No, you aren't, and it isn't vocal fry. Overtone singing is a distinct technique in Tuvan throat singing, and comparing it to a bandpass filter is accurate — as a whole, Tuvan throat singing is a set of techniques designed to induce vocal sounds with extremely rich harmonics, which can then be modulated and selected for by shaping the mouth.

What you're thinking of is called "Kargyraa", a particular subset of Tuvan techniques that involves singing with the vocal cords as normal, but also tightening the voicebox such that your "false vocal cords" (some flaps somewhere in your throat) are struck at a frequency an octave below the sung note — for every full cycle your vocal cords complete, the false vocal cords complete half of 1. It creates a rich sound which can be useful for the "bandpass" technique, but is fundamentally something different.

Take this with a small grain of salt, I came to this technique through the modern beatboxing community who independently discovered it as the standard "throat bass" and only bothered researching the Tuvan equivalent a long time ago.

There is a bass singing technique called "subharmonics" that uses something similar (identical?) to vocal fry to create interference with a sung note for a similar effect.