I think that’s the reason

I've been thinking the same thing since WandaVision came out. There is also personal and news footage that would circulate online. Even word-of-mouth would spread from eye witnesses.

Either that, or The One Who Remains gave them the time theater footage.

I kid.

I think it's just another thing the wandavision writers did not think through.

Agent coulson had to debrief them obviously 😂