Technically it is a 3rd party app. Comcast does own NBC but they are separate legal entities. If you pay for Peacock Premium Plus or whatever they call it to eliminate ads via Xfinity a portion of the $5/mo. for that goes to the Xfinity affiliate and the rest to NBC. Therefore NBC is treated by Comcast in a more similar fashion to other competitors like ABC and CBS etc. Service providers and media companies are still heavily regulated compared to many other industries and the affiliate transaction model still exists to help keep holding companies from subsidizing money losing divisions by funneling unrelated profits from other areas under the parent company umbrella to unfairly steal market share.
All that boring legalese aside, if I hit the Info, Guide, or On Demand buttons on my remote and the app completely closes out then it's 3rd party as far as I'm concerned. Same thing if I can't add a movie/episode to a playlist. The only app that acts like a linear channel where you can sift through menus and add content to playlists while the app still runs in the background is Sony Crackle which is weird if you ask me. I thought that Peacock would be more tightly integrated with X1 but boy was I wrong about that. That's what I dislike the most about most apps on X1. If I accidently sneeze too hard near the remote or my cat rubs her stupid face on it the app may completely exit out with mixed results when it comes to resuming where you left off.