It happens all the time. Even with things that seemingly arrive 'out of the blue'.
Both the Germans and British invented jet engines within months of each other. They also both invented radar at around the same time.
I have invented things that were later patented by others, too. This was my most important, I invented it about 10 years before it was patented, but (stupidly) did nothing with it: https://www.youtube.com/watch?v=CjTa3SeKuaE
Millions of people have the same background knowledge that you do. Any one of them could have done the same as you. The thing about one-in-a-million chances is that when you have ten million opportunities you will very likely get ten hits.
When did you actually see that it was filed? If you saw it come out of the patent office a few days later, it was probably filed much earlier than your forum post.
If you saw it later but it was filed several days after your post, I think it is feasible, perhaps with the aid of someone like GPT-3, to put together a patent application that quickly.
Edit to answer your first question this happened between Nov & Jan.
Was your post archived at archive.org prior to that patent being filed? Or is there some other proof that you published before they filed?
Note that their claim being invalidated does not mean you get to patent it, since you did not file prior to publishing (via your post).
They may also have been working on the same thing at the same time as you, and then needed to accelerate their patent application after your MVP posting.
The only thing that really matters is the claims section of the patent, so if you can access those be sure to read them to determine how much it overlaps with your stuff.
When you meet with the patent attorney you should ask them how you can establish/archive prior art for your previous post on the MVP and also ask them to go over the claims for you. Typically the claims are written in a very general fashion and then they will get more specific sub claims to narrow the patent. It would probably be useful for you to know how narrow the claims are.