single player only and for someone who has no idea of the original material, board game nor books/tv series.

Came here to post same! Really hope the singleplayer AI is good.

AI is pretty good.
That said you do miss the experience of talking to a real person and lying/peddling/allying to his eyes.

You do have a system to propose alliances and request support from AI but it's not the same ofc.

The game requires a "broadband internet connection" (GOG store page), and, according to Steam, a third party account.
Is this true for playing alone, too?

To require an internet connection for playing alone (not to mention a third party account) would violate this GOG statement (to be found on this game's GOG store page):
"Why buy on GOG.COM?
DRM FREE. No activation or online connection required to play."

The broadband connection and third party account is required to play online multiplayer. From my very limited experience with the game as Epic was giving it away for free, single-player doesn't seem to require this.