He was talking about tevel d'rabanan -- food that requires tithing, but only rabbinically.
It's a pot that's just self-sustaining. It could be suspended. Hydroponically, even. Maybe, i don't know, it's a little different. It's liquid, not dirt. Who knows. (approximately 47 seconds into the video)
So, indeed, who knows? Or in Hebrew, mi yodeya? ;)
Would hydroponically grown plants require trumah and maaser derabbanan?