Also, have re-read the rules in the main rulebook talking about detachments. Check this out:
"Other Force Organisation charts are available for players to use, both in this book and in other Forge World publications, and some Army Lists may present specific variants for use with that list. In all cases, these charts will adhere to the same set of basic principles."
"Sometimes, a single choice in a Force Organisation chart may allow you to select more than one unit, or to vary the Battlefield Role of the unit selected. In all cases, such deviations form the normal procedure will be fully explained in the Army List or publication that presents such a Force Organisation chart."
Not only do both of these statements acknowledge the existence and relevance of other detachments in other books, but it also states that there may be exceptions, special circumstances and differing procedures with these detachments in other books or army lists. This is clear as day with the wording in the legion Astartes army list:
"When playing Horus Heresy games using these alternative charts, the Rites of War rules for the Space Marine Legions may not be used in conjunction with them unless specitcally mentioned in the mission, as the provisions and formations illustrated by the chart take precedence over them."
This is a special circumstance in an army list publication, and would therefore fit the description outlined in the first two snippets from the main rulebook.
TL:DR army lists give specific exceptions/rules to the optional force charts unique to their own army but this does not get overruled by the main rulebooks 'generic' ruleset for them.
Edited by The Ironic Warrior, 07 February 2020 - 12:58 PM.