The only
HH FAQ is this one:
https://www.forgeworld.co.uk/resources/fw_site/fw_pdfs/Horus_Heresy/Horus_Heresy_7th_Edition.pdf
As noted, in Heresy games only troops (not including dedicated transports) and units with the Implacable Advance rule are scoring.
The confusion is because in addition to their own army lists, the
HH setting also has their own missions.
If you're playing a 'normal rulebook mission' then you can still use a
HH army - just use one or more detachments from the rulebook, and follow the rules normally. In
HH scenarios (like Shatter Strike) then follow the
HH rules - only a single Age of Darkness Detachment, 25% limit on lords of war, only troops scoring, etc.