To the best of my knowledge, there was no reliable evidence that the core members of the Fontan Lions were multicheaters while they were known as such.
However, after their leader ragequit and they followed, there was a great deal of suspicion. Some time later, he rejoined, as did a few of the other accounts. At that time, they were, in fact, determined to be multicheaters, and their new accounts were locked as such.
It is, of course, quite possible that originally, there were several players, and that either before the original ragequit or at the time of the new accounts' creation, the leader took over their families. However, it's also possible he was the only real person behind the Lions from the start. At this point, it's impossible to know for certain...and not really all that important.