score:8
This isn't, so far as I can tell, any kind of travel jargon. It looks the standard definition of in-house used in any kind of business:
Done or existing within an organization
That is, if you want to book a room directly through the hotel itself, use the contact information provided. Presumably, you could also book the room through a third party like a consolidator, online travel agency, or corporate portalβ which might or might not be cheaper and/or easier.