The intention was almost certainly that it was a 5M. When the pack was written, they used modern refits to represent star league mechs with advanced technology.
(sometimes with no regard for continuity, such as using the Zeus in that lineup even though it didn't exist at the time, or an earlier scenario where Manticores and PPCs appear literally centuries before they existed)
If you want something more "authentic" to the Star League era, you could substitute Royal mechs for the mechs listed as post star league designs. So the stalker, for example, could be replaced with the STK-3Fb variant.
If there isn't a royal variant, you could instead use the original, or stick with the variant listed and call it good.
And for the Zeus (since it wasn't supposed to exist yet) you might consider swapping it for an appropriate star league era design. I'd suggest the Rampage, since that is a Rim Worlds built assault mech.
You might also consider letting the Black Watch have royal mechs as well, since they're supposed to be the elite of the elite.
