There may be something illegal about letting the non-players out there on the field... but that would be picking nits.
The only restriction here is that the warmup period must only be 1 minute (which is pretty dang short), and that once the umpire begins the inning, the pitcher has 20 seconds to throw a pitch. The team could have been in violation of either of these... but a great many umpires do let far too much dawdling go on between innings.
Had a local team that HAD to huddle before each half inning. Often, their pitcher got zero or one pitch to warm up, but it was their choice how they spent their minute.