AFAIK, briefing is a part of a geoscape layer =)
It looks like the game does not respect anything from the mission, it only takes `data` section from alien deployment and race from mission. Briefing data looks like hardcoded part in case of base defense mission.
How do you think, is it a place for some improvement? It is not very immersive in some mods to read briefing about an alien vessel landed near the base and then fight rats, coming from the ground or cultists, travelling on the vans or helicopters...