Dude, since there were no biplanes around to 'write' in the sky with puffs of smoke, odds are pretty good that the phrase you quoted above is - how shall I put it - something called 'poetic license' on the part of the author....
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.
There will never be enough proof to quiet the sceptics. You are shown the proof of a miracle in the sky, and now you ask for proof of the writing in the sky. It's hopeless to persuade those who will not see.
Also, who needs biplanes, when you already have airworthy chariots of wheels within wheels (Ezek), flight capable angels, and ultimate power? Not to mention the ability to affect men's senses to cause them to see what you want them to see, like Jesus did right after the resurrection when he was talking to the men about the recent goings on, and he prevented them from recognizing him until he was ready.