I noticed that the briefing text displayed when using ctrl+B in battlescape is not the one specified in the ruleset, but always the default one.
In other words, this ruleset will change the briefing text for when staring the mission, but not the one that is displayed using ctrl+B:
alienDeployments:
- type: STR_HARVESTER
briefing:
desc: STR_BRIEFING_TEXT_HARVESTER