This is farcical. They didn't have to errata the Cargo Hold and Lighter Bay to allow Grand Cruisers to take it for the same reason they didn't have to errata the Vitae-pattern Life Sustainer to allow Grand Cruisers to take it!
The clear intent of the rule is "may use any component that a Cruiser could."* The strict reading of "cruiser-only" would restrict you to "All Ships", "Grand Cruiser", or exclusively "Cruiser"- and there are very few indeed of the latter. In fact, doing so would eliminate:
- all warp drives
- all Bridges except Ship Master's, Invasion, and Fleet Flag
- all life-sustainers
- all crew quarters
- all macrobatteries (!) except the Hecutor-Pattern Plasma Broadside
- the Titanforge and Star-Flare lances
- all lance batteries except the Godsbane
- the Jovian-Pattern Escort Bay
- all the "standard" torpedo tubes- Fortis-, Gryphonne-, and Mars-patterns.
- and literally countless supplemental components that it would be absurd to exclude, e.g. Munitorium.
thereby rendering it impossible to build a Grand Cruiser at all.
I think the error in your logic is that you interpret "cruiser-only" to include both Cruisers and Light Cruisers, and thereby permit Grand Cruisers to equip "Light Cruisers, Cruisers" parts such as the larger versions of the life-sustainers and so on. However, the entire rest of the rules treat Light Cruisers as their own class, not as part of the general "Cruiser" category for any other purpose. Even where a clear rules commonality exists, such as in the rules for prow weapon arcs, the classes are called out individually: "Prow weapons on light cruisers, cruisers, or larger vessels may fire to the fore, port, and starboard." (RT p.219).
The strictest reading of the rules text is clearly broken and not as intended. It is far more consistent to consider the rule to permit "any component whose restrictions include Cruisers", rather than "any component whose restrictions don't include anything EXCEPT Cruisers".
*This is, itself, rather odd, as several Grand Cruisers are called out in their fluff descriptions as out of production because the Imperium no longer can- either "at all" or "practically"- produce their plasma drives. The rule really OUGHT to allow any Cruiser component except the plasma drive, or- in keeping with the precedent set by the Conquest-class Star Galleon- any Cruiser component, but if a Grand Cruiser version exists, they must use the Grand Cruiser version.