LOL, yeah, poetic license on the part of Constantine and his entire army. Considering that the army marched against odds and conquered, and it appears to have made a very dramatic change in Constanine's life, I'd say the odds are much greater that this wasn't a chance meteor with poetic license but was a true divine intervention complete with Holy Sky Writing.
People like Saul who are on the fast track to religious leadership don't suddenly have Damascas road conversions and become one of the people they are killing, enduring stoning and prison, unless something really happened.
A leader like Constantine doesn't suddenly embrace a new religion like Christianity as a motivational technique when most of his men aren't Christian. No leader would introduce an unpopular religious controversy right before a battle. But when the whole army sees the vision, they can kick butt, and there can be changed lives.
Yes, the sight of a fireball followed by a nuclear-like explosion indeed qualifies as a vision....no need to try to embellish it...