It's difficult to find the text of the memo, but from what I can find a monitor in the hallway is explicitly against the rules.That’s sort of my point. It seems some players and coaches may have thought (incorrectly) that the prohibited action was electronically transmitting information obtained from video to the dugout, by AppleWatch or phone, for example. And that manually transferring the info from video monitor to players in the dugout or on the field was okay, if only because “everybody does it.”*
"No television monitors are permitted in the tunnels or auxiliary rooms between the dugout and the clubhouse."
https://www.si.com/mlb/2019/02/19/major-league-baseball-sign-stealing-rule-change