Matthew, I do not know you, so I do not mean to insult you, but if you check more closely, you will see that 14 of FLJ's air-to-air victories were captive observation balloons. When a balloon goes up, everyone in the next three to five adjacent counties will know about it. The pyrotechnics are rather obvious.
The remaining four fixed-wing a/c he bagged were pretty much confirmed (one was a "share" with
Joe Wehner I am told), and the additional "three" unconfirms do not come into the official equation.
Even in the worst possible estimates, Luke was the something of a "baddass" if you know what I mean. I personally do not doubt totals of 18, 19, 20, or 21. Any of those scores are believable, in my opinion.