Additional context to possibly speed this answer along:
It has been suggested that this errata note for primitive jumpship construction...
"All armor mounted on a Primitive JumpShip—including the “free” armor provided by the vessel’s structural integrity value— must be multiplied by 0.66, rounding down to the nearest whole number."
...would be applied to all armors of any type, including those more advanced types available to retrotech jumpships.
However, the text refers specifically to a primitive jumpship, and matches references to primitive armor found on primitive battlemechs, primitive aerofighters, primitive dropships, and primitive small craft. For example armor on primitive dropships says the following:
"Primitive DropShip armor is identical in all ways to standard armor (including costs), except that all armor mounted on a Primitive DropShip—including the “free” armor provided by the vessel’s structural integrity value—must be multiplied by 0.66, rounding down to the nearest whole number."
This is almost identical to the above errata note about armoring primitive jumpships, save omitting any reference to specifically discussing primitive armor.
In addition, it is already established on page 121 of interstellar operations that:
"Primitive ’Mechs, aerospace fighters, small craft, and larger aerospace units (including DropShips and JumpShips) may only mount Primitive Armor".
This is repeated in each of the other unit's sections, but not in the primitive jumpship text in the un-eratta'd version. It's my contention that the purpose of the errata mentioned at the start of this post was to bring primitive jumpships in line with other units, in essence restating that these primitive units are limited to primitive armor, not to apply an additional modifier to retrotech hulls as well.
To further support this, whenever retrotech units are discussed in any of the other rules entries, they are done so in a section clearly marked as such, to indicate the difference between primitive units and retrotech units, which the book identifies as distinct unit types. The errata specifically refers to primitive jumpships.
To sum up, the 0.66 modifier is clearly talking about primitive armor on a primitive jumpship, because it specifically mentions primitive jumpships and primitive jumpships can only carry primitive armor. Retrotech jumpships aren't, by the rules, primitive jumpships and can mount any armor type (page 122):
"RetroTech units may mount any type of armor available to the unit type, including modern armors such as ferrofibrous, hardened, and the like."
Therefore that modifier would only apply to their armor if they were using primitive armor.
At least that's my assumption. Thank you for coming to my ted talk, I'm going to go ice my wrist now.
