Hi Jonathan!
So sorry for the late reply!
Really glad that you're part of the site!! :)

This is a great question, and many players are wondering about this.

The reason for anchoring the pinky on the pickguard is for stability and so that you get a steady right hand that is not floating around too much when playing. Many people do this - such as the players you mentioned, but also Chet Atkins and many other great fingerstyle players.
The big advantage for this is that you can bring up your hand from the bridge - so that you don't mute any strings, but still get a steady hand. For instance, watch Tommy at 4:28 in this video, where he plays a open string run (in the song "blue moon") with all strings ringing out beautifully - but he still has that steady hand floating over the bridge - because he is able to use his pinky to have a real steady hand even though nothing is muted. https://www.youtube.com/watch?v=sZA7g0A7asI

Some players rarely do this, like myself. I get the steady hand from having my palm on the bridge and/or from the right arm against the top of the guitar, so to speak. So when I don't want to mute the strings and play an open run for instance on the lower strings - it's a little bit more difficult for me to have a real steady hand without muting anything.

So I'd say that using your pinky on top of the guitar can be a great thing! But if you're like me, that might not feel natural to do - and that's ok too! :) Some players rarely use the pinky on top of the guitar and sound great anyways, so it's all about finding your own way of playing that feels good. Try practicing an open string run using most strings - and see if you can get your hand real steady without muting the strings. That's a great way to work on this!

Hope this helps!
Kind regards/Emil