Sacred Haven of Debauchery

But you're not explaining why I should.

Because you decided to show off your grammar skills and pointed out a supposed mistake, even without being completely sure. If that's the case, then you have a reason to suspect this mistake. So, you should state those reasons before I disprove them, you chubby fellow. The relevant rule of the Russian language is there, and so on.

That's 100500% an introductory word.

You can use it, or you can choose not to. But yes, it's an introductory word. They are not always separated by commas.

And this is like an idiom, although I'm not sure
more precisely, it's a modified idiom, of course))

It's not even close to an idiom. An idiom is something completely different.