Homeworld Fleet gets owned by pop cap.
Also, no shields, Holy Imperium of Man has void shields. Homeworld tech is obviously superior in other regards, as they understand what the hell they are doing with it, but if they run into a proper battlefleet they are proper fucked.
As far as production times go, yes, they can almost certainly build faster than the Imperium can, but not in 'video game time'. If they are not crushed outright in a decisive fleet action they lose by attrition anyway.
Or they just become a pirate nuisance until a SM chapter launches a crusade against them for plundering the worlds under their ward.
Oh, and Terra has ungoldy defenenses, not to mention Luna which is more or less a giant weapons platform.
Teleporting teriminator squads on board the mothership also yields doom.