The bonus was
7 euro free bet for 777 users. That's 5439 euro, minus the house edge because of wagering requirements. Even if it's abused, a few thousand euro shouldn't be a problem for any casino.
Unless there's literally no money.
So what I think is that the excuse was bullshit, because with the wagering requirements, I think it would be less than few thousand euros, no matter how much abuse there was, and if you make a promotion like that you have to have it backed with funds.
Surely they were very short of bankroll, seeing the amounts that NotATether mentions, who seems to know well how the subject goes.