The technique described here would work, it’s a Salzedo thing actually, but that’s not what is happening in the video because he is clearly not muffling some notes while playing the rest.
Also in the Seguidilla from Salzedo’s Suite of 8 Dances there is a similar technique where you do gushing chords (very fast glisses in the span of a chord) on specific notes first with the second finger and then with the third finger (going up) and using the thumb to go down. This is also in Chanson dans la Nuit. Again not in this video.
The technique used in the video, as far as I can tell, is actually just glissing rapidly (like the gushing chords) up with 2nd finger and down with the thumb, so you’re actually doing a turn-around gliss very, very fast.
Do you know what I mean? Do a short turn-around gliss going up with the second finger and then back down with the thumb, no space between the sound going up and the sound going down. Then just keep doing it faster until it sounds like strumming.