Players, misidentified threat, dead set on wrong assumption

So I'm doing a fallout style campaign, and the overarching threat I'm hinting at is something a little unusual. Ill lay out the clues first then tell you what they thought.

A psychic seer on the radio speaks of a dark sun, a shambling terror and ionizing rays showering the land.

They're on watch at night looking over the horizon at what they think is a radio mast at first, but then they notice it's swaying. The light at the top multiplies, from one to three forming a downward triangle while the orb of light in the middle grows brighter. They use a telescopic lense to get a better look and see a very large(200ft tall) not exactlty bust mostly humanoid shape.

Radios and other electronics capable of sound and image emit massive amounts of static, underweight is a long groaning moan.

They go to wake the others and the whole ass thing is gone. There is no radio mast, the "radio mast" walked away.

They assume it is the Zetan Aliens, cause I guess the radio thing was a little like the baby monitor scene in Signs and I used the signs soundtrack for music but just cause it's creepy.

On the road they go through a town that was very obviously devastated by high levels of ionizing radiation that left a clear path, even including the asphalt melting and being churned up in large sweeping potholes(like footsteps) and many inhabitants being flash ghoulified.

One single half feral survivor speaks of how it ***walked*** and how those that could followed it, this dark sun, bathing them in its glow. Still they assume aliens, flying, in a ship.

What I'm trying to hint at is something like the Maypole an impossibly giant ghoul, but they are so dead set on it being aliens I'm not sure if i should just scrap it and settle for aliens. I've given them other hints, even an image to go with the night watch scene, and they still insist it's aliens, getting frustrated even. So like wtf? Did I just not describe things well, or...fuck I don't know. Should I just out and tell them what it is? They know things get giant like the super mutant behemoths maybe they just think things can't get *that* big? Should I just play it off like "it was so gargantuan and terrifying your brain broke and made you think it was aliens"