Throwing a pass while falling by no rational, objective judgement would deem a player to be a runner.
I think your confusion is 100% sincere, so I'll try to explain as best I can.
The confusion is in the terms "runner" and "receiver." These terms have nothing to do with the position you play, or whether you're running down the field or dropping back to pass. The NFL uses these terms only when it comes to questions of possession. A "runner" is simply a player in possession of a live ball -- a ball that's in play, between the snap and the whistle. That's it.
At the snap of the ball, the QB is the runner. Whoever he hands it, pitches it, or passes it to...that player then becomes the runner.
BUT...the player at the other end of the pass (the receiver) doesn't become a runner simply by gaining control of the ball.
First, he has to 1) get control of the ball, then (or simultaneously) 2) get two feet down, then 3) maintain control long enough.
Then, and only then, does he establish himself as a runner.
The part in bold is called the "catch process." Without it, there would be no way to determine how a receiver becomes a runner. If a player is in the middle of the catch process -- let's say he has control and one foot, or control and just got the second foot down but hasn't maintained control long enough yet -- then he is
in the act of catching a pass. He's still a receiver. He's not a runner yet. He's not in possession of a live ball yet.
Item 1 says that a player going to the ground
in the act of catching a pass (a receiver going to the ground) must maintain control of the ball throughout the process of contacting the ground. Item 1 exists because when a player dives for a ball, or goes up for a ball and only manages to get two feet down before his body hits the ground, he doesn't have time to complete part 3 (maintain control long enough). There's no
time for him to become a runner. So he has to maintain control of the ball after his body hits the ground.
How does an official know when the player has had enough time to become a runner, after control and two feet down? This is where it forks off into two very different directions.
Prior to 2015, the official knew the time requirement had been met as soon as the player did something that only runners do. It's common sense, after all, that the thing that would determine that a receiver had maintained control long enough to become a runner would be to do something that only runners do. This is the football move, and since players routinely make football moves while falling, it didn't occur to anyone to make a rule that stipulated that a player must be upright in order to make a football move. Again, common sense.
Then something happened in Green Bay in the playoffs following the 2014 season.
And in 2015, the standard for meeting the time requirement to complete the catch process and become a runner was changed. The football move was completely removed from the rule book, and the standard for becoming a runner was now that a player must remain "upright long enough." How upright? For how long? Nobody knew. The call would depend on the field official's judgement, and could be overturned based on the replay official's judgment, with no regard for what used to be the catch process.
After one year of this, the commissioner appointed a catch committee to "clarify and streamline" the catch rule. Their recommendation was to put the football move back into the rule book, and this time with specific examples. Unfortunately, the standard for becoming a runner remained (and still remains) the same. A player must be "upright long enough." How upright? For how long? Still, nobody knows. The call still depends on the field official's judgement, and can still be overturned based on the replay official's judgment, with no regard for what used to be the catch process.