It didn’t have to be “realeased”. Just one guy who was infected in West Africa or who picked it up in say, one of the “sexual networks” of the guy who in the UK who brought if back from Nigeria (could be second or third- or fourth-hand), could spread it to dozens at an event like that. Then those guys go back to their home countries and attend an orgy at a sauna (Spain) or a similar event. It appears to be occurring in a highly promiscuous segment of the gay population who engage in unprotected and extremely risky sexual practices involving the exchange of bodily fluids, who are attracted to super spreader events of these types. No nefarious plots or grand conspiracies needed.
Everything you say is true and perfectly reasonable.
One terribly significant problem is the fact they left their game plan laying on the table. And I read it. (you can too, if it’s still at the website!)
It is simply inconceivable to me that this is a naturally occurring event.