If I said that, I apologize! I may have been glib in my wording, but pick depth of course definitely matters - for lots of reasons, from tone and attack to picking motion. I think what I have objected to in the past is this idea that we had in the '80s that by attempting to use the smallest possible amount of pick on the string, it somehow solves all these tangential picking problems, like switching strings, hand synchronization, and so on. There was a time when any question about picking technique resulted in “choke up on the pick”, “use less pick”, “use smaller motions”, no matter what the actual problem was.
That being said… we can see from the closeup footage we have filmed that most players do use a relatively small amount of pick on the string. This includes players who are still learning, and who post in Technique Critique. It’s actually remarkable how small an amount this can be, and how players can modulate this by feel. Controlling your hands to modulate between 1/8 to 1/16 of an inch of the surface of a moving object to contact a vibrating string is an impressive feat of fine motor control.
And I would note also that this is not the same as having a lot of grip exposure. The amount of pick sticking out of your grip helps you reach the strings - the amount that hits the string is typically way smaller:
The good news is that most people seem to learn to do this “small pick on the string” trick. And as you’re discovering, you can modulate it too, maybe without even thinking about it based on changes in other aspects of your technique like your motion. There is not a whole lot of evidence that “using too much pick” is a problem that’s holding many players back.
If your attack feels unsmooth, there are other factors, like edge picking and grip flop that come into play. And these dovetail with the gauge of the pick you’re using. You can grip a thin pick pretty rigidly and play it face-on with zero edge picking, and still move through the string. Try that with a 2mm pick that has no taper on its face and you probably won’t get through.
If your motion itself isn’t working in some way, that’s a whole other factor. Lots of interplay here. But again, apologies, because all these things “matter”!