That is really cool, Just when I thought I was getting pretty good at finger picking. I don’t even use a thumb pick, but it looks like this particular way of playing harmonics is easier with a thumb pick to help avoid deadening the ringing open strings.
I don’t use a thumb pick either and I usually do straight natural harmonics using the fingernail on my index or middle finger. But to do these harp harmonics, unless you have a thumb nail that sticks out like a sore thumb, you are going to need a thumb pick. The thumb pick also moves the picking location back about a quarter of an inch (which up there is about a half a fret) so you don’t choke the note.
I hope to learn how to do this stuff someday. I guess you just have to practice it over and over until you can do it in your sleep.