Yea, so? The arachnid guardpost is supposed to be crazy strong against starship class or above stuff. When faced with crazy strong attacks, it makes sense that armor is not enough to deal with it.
That's like saying "Oh man, I upgraded my Seige Tank's armor to 3, but it still dies in one hit to nukes". (Let's ignore the fact that nukes ignore armor in SC2 for the moment, pretend that it did pay attention to it)
Or are you saying that the armor value of the Spire Battleship's armor is too underwhelming given its "semi-superweapon" status? Or are you saying that the Arachnid Guardpost line is OP? Or are you just pointing out something that is seemed interesting to you?
I ignore armor normally, and would have just said, "it kills it in two hits," but that would be technically incorrect, because the armor brings the guardpost down from "exact damage" to "within a rounding error".
I suppose my problem is that superweapons just seem to die too easily.
A spire Battleship costs about 1.7 million M+C with modules, and 21k knowledge, and exo-waves, and a bunch of other hassle, while a Zenith Starship mk3 is faster and has more durability for 800k M+C and 4k knowledge and no other hassle.
EDIT: The more I think about it, the more I find myself convinced that superweapons need to be at least an order of magnitude more powerful than they are now (relatively speaking, so removing a zero from non-superweapons that doesn't get removed from superweapons is an action that meets the criteria)