It is a staggeringly stupid thing to do.
Footballers are paid an extraordinary amount to live the life of professional sportsmen. For example, this means that they train and travel on Xmas Day instead of getting pissed and eating a turkey dinner. They get a couple of months every summer to act like carefree young twats. For the rest of their time, I expect them to act and behave like professionals.
Over and above this, when your club is at the very edge of catastophe due to unavailability of players, it is even more important that you take your wellbeing seriously. Spending an evening in a confined space with hundreds of others is utterly mad.
If they haven't broken club rules by doing so, then the rules need to be changed. The only possible rationale for what they did was to deliberately go and get Covid because the club want their next few matches to be called off.