I don't have any inside info but perhaps they were playtesters, perhaps they have played the game at a convention, perhaps they were in on the development. There are ways folks can play a game before it is released....and sometimes you don't even have to play it:
The D&D adventure system board games more or less all use the same rules, so if you've played any of the previous entries, you'll have a pretty good idea what to expect.
E.g. my ratings of the previous four games are only a half point apart.
And if you've seen the previews and maybe read an article or two about it, you'll also know about most of what they've changed.
Imho, there's also nothing wrong with adjusting you rating a bit after you've actually played it.
But in general, I share the OPs dislike for the 'wishful-thinking' advance ratings that are all too common, especially for kickstarter games (and the inevitable '1' ratings to 'balance' the over-excited backers).