In 2018, HP Umpire Dan Bellino "caught" a pitched ball with his armpit in Colorado and awarded baserunners a base each as the ball remained out of play, and this might be where some of the confusion comes from. Official Baseball Rule 5.06(c)(7) supports this call...for an umpire trapping the ball: "The ball becomes dead and runners advance one base, or return to their bases, without liability to be put out, when—A pitched ball lodges in the umpire’s or catcher’s mask or paraphernalia, and remains out of play, runners advance one base."
For pitcher Alexander, however, OBR 5.06(c)(7) does not directly apply (it is a batted ball, for one), so instead we turn to the MLB Umpire Manual which clarifies that, yes, similar (but different!) lodged ball rules apply to infielders, too: "If a batted or thrown ball inadvertently goes inside a player or coach’s uniform, lodges in the catcher’s face mask or paraphernalia, or is intentionally placed inside a player’s uniform (e.g., in a pants pocket), the umpire shall call “Time.” The umpire will, using common sense and fair play, place all runners in such a manner that, in the umpire’s judgment, will nullify the action of the ball going out of play."
Video as follows:
Alternate Link: Pitcher caught a ball with his armpit and how this differs from an umpire lodged ball
The primary difference (other than pitched/thrown/batted) is twofold. First, the Colorado play involved an umpire not a player interacting with a ball, and second, the MLBUM interpretation requires the ball actually go inside a player/coach's uniform or lodge in the catcher's equipment.
Because the ball never went inside Alexander's uniform, it was accessible to the player, and (if you want to take it step further in OBR 5.06(c)(7) language) did not remain out of play, it is considered in play and a legal catch.
Video as follows:
Alternate Link: Pitcher caught a ball with his armpit and how this differs from an umpire lodged ball
