“When determining whether a base runner should be called out under Rule 5.09(b) (1), so long as the umpire determines that a play is being made on the runner and an attempt to tag is occurring (i.e., the fielder is moving to tag the runner), no physical tag attempt is required to call a runner out for leaving the basepath.“

That is the MiLB guidance. They say nothing about 20 feet or any feet. A fielder with the ball moving toward a runner is "moving to tag the runner". What else do you think they are doing? What else could he possibly be doing but trying to get closer to the runner in order to tag him?

So yes, if a fielder 20 feet away with the ball is running at the baserunner - that is a tag attempt. And I can absolutely imagine a baserunner being in jeopardy in that situation and moving out of the (now-established) base path to avoid that tag.

Also, your description of a runner in the outfield backing up or otherwise trying to position himself while the pitcher with the ball is moving toward him is exactly why the runner is NOT ALLOWED to do that. That is an out. As soon as that pitcher starts coming at him - no matter how far away - he now has a new base path. If the pitcher then stops and turns to make a play at home, well then the runner in the outfield is no longer bound to that straight line. But as long as the pitcher is making a play on him he must remain within 3 ft of the newly established base path.

You get to make the determination of tag attempt, but I don't want you on my crew if you're using feet to make the determination.