So here's the relevant rules, as I see them:
The outcome if B did not touch the ball (doesn't seem to be any disagreement on this one):
Ball Dead in End Zone (6-1-7-a)
ARTICLE 7. a. When a free kick untouched by Team B touches the ground on or behind Team B’s goal line, the ball becomes dead and belongs to Team B.
If there was no touching, that would be the rationale that the ball is dead and belongs to B.
But if B did touch it, but did not posess it, it is a muff: 2-11-2
ARTICLE 2. To muff the ball is to touch the ball in an unsuccessful attempt to catch or recover it. Muffing the ball does not change its status.
6-1-3-a
ARTICLE 3. a. No Team A player may touch a free-kicked ball until after:
1. It touches a Team B player (Exception: Rules 6-1-4 and 6-5-1-b); criteria met: yes
2. It breaks the plane of and remains beyond Team B’s restraining line criteria met: yes (Exception: Rule 6-4-1 (interference with Opportunity, not relevant) (A.R. 2-12-5-I (breaking the plane of the restraining line, not relevant); or
3. It touches any player, the ground, an official or anything else beyond Team B’s restraining line. criteria met: yes
Thereafter, all players of Team A become eligible to touch, recover or catch the kick.
Based on the (supposed) touch(muff) by B, this establishes that A players have a right to the ball. (only one of the criteria must be met)
Once B touches it, the clock should stat (on an official's signal): (3-3-2-a)
Starting and Stopping the Clock
ARTICLE 2. a. Free Kick. After the ball is free-kicked, the game clock shall be started on an official’s signal when the ball is legally touched in the field of play, or when it crosses the goal line after being touched legally by Team B in its end zone. It is subsequently stopped on an official’s signal when the ball is dead by rule. (A. R. 3-3-2-VII)
So we have kick -> touch by B -> clock starts which means the ball is live until possessed by A or B, and dead by rule (player down, through/out the endzone, etc)
note: I have not seen the entirety of the play (just Calhoun's gif, which sure looked like a touch to me.) So, based on the description of the series of events here, and what I see in the rules, A should have had an opportunity to recover, since it was a live ball, even in (but not out of) the end zone. But, if there was no touching, it's a moot point. This isn't intended to be criticism of the officials on the field AT ALL. Their job is incredibly difficult and tbh i'll probably never be good enough to be a college-level official, at any level.( I WANT to be that good, but let's not kid ourselves, I'm also a realist here) I'm just trying to get better at learning the rules, and walking through a scenario like this is immensely helpful for me.
So, to that end, please let me know what errors I have made in arriving at the conclusion I did (I'm sure there are some), and if I missed any relevant rules.