1: 6vis means 6 visible genes, right?

2: What's the big deal with 6vis? I've seen people posting 'I GOT A 6VIS' everywhere!

yes, 6vis is six visible traits

It's a big deal because it takes a long time to breed up to 6vis if you're doing a Project, especially if you are after a very select group of traits. "A long time" as in, it can (and typically does) take more than a year, depending on how nice the rng is feeling.
If you're starting with essences then the max TP you can start with is four (while a 6v has 12)
Exclusives can have a max of 3vis (and making 3v exclusives is expensive, so it's often cheaper to just start with a larger pool of 2v)
Wild caught themes (as with all wild pets) can have up to 8tp, but mythicals are not something you just come across every day (especially not themed ones), and the typically what a person has to work with is pets with 2-4tp, maybe 6.
(and, again, if you're after very specific traits, doing theme projects with exclusives or wild themes suddenly becomes more tricky)

(example: it took us around 400 days to get from a starting stock of 4 3v exclusive themes (here's one of them) to our first 6v (this is he)

So people being highly enthusiastic about having gotten to 6v? Yes, they're going to be happy about it and they are ABSOLUTELY allowed to be. That's their time and effort and dedication and gold and diamonds in sylesti form.

Wow. Okay. Thanks!