The memo isn’t first hand in any way. A lot of things get confused over distance and time, the memo was dated in 1950, but the event happened in 1947? I’ve always heard that everything involving silver discs around Roswell was related to Project Mogul. What probably happened is someone saw a crashed vehicle from Project Mogul with crash dummies inside (maybe small, human-shaped crash dummies.) The officer who observed it wasn’t necessarily supposed to, but as often happens with secret projects in the military, not everyone is going to be privy to it. It’s entirely possible someone who was part of RAAF saw a crashed test vehicle from Project Mogul who wasn’t supposed to see it, saw some dummies and made conclusions. The people running Project Mogul weren’t about to clear things up, that would have violated the law and served no purpose.
If it had been bigger news at the time, it probably would have come out to allay public concerns. Since Roswell wasn’t really big news until some 25+ years after the fact (in the 70s), by that time it was easy to just dismiss the whole thing as a hoax. Since it was but a blip in the news and quickly explained as a weather balloon, and because in 1947 no one dug deeper and the public mostly never even knew it happened, things became mysterious. If, for some reason the crash had been a huge deal in 1947, I think there would be far less mystery today. If it had become a sensation eventually someone higher up (maybe even Truman) would have said “fuck, just tell them it was an experimental military vehicle and end this.”