Anchoring a finger on the string (such as the a finger on the first string) is common and widespread among concert level guitarists. It's really only teachers who don't know better who are spreading bad information claiming this is a bad practice. I've seen Manuel Barrueco, Pepe Romero, Philip Hii, Ana Vidovic and many more doing it. Thanks to you I can now add Iznaola to the list.
Here is Ana Vidovic bracing the a finger during tremolo
There are many more video examples of the finger anchoring on this thread
An eyewitness will often only see what he already believes to be true.