I don't believe that is correct. You may want to check into that further.
I may be wrong, but I have found two interpretations of the rule. One said a candidate has to be on a ballot, the other said that any “legally qualified candidate” would trigger the rule.
The networks tend to go by the more conservative, from what I understand. This is why cable channels usually stick close to the rules so as not to invite regulation. The HBO movie that Thompson is starring in during the latter half of May might be a factor, as well.