Medusa technically isnt a Heroic Spirit, she's an Anti-Hero. Also no, she doesn't have that, she has a Servant. Anyone who had their very own Heroic Spirit could conquer the world.
Well, OK, but it's still pretty impressive to get someone who could crush you like an ant to devote their existence to protecting you (and without even
trying, in Sakura's case)....
Also it's really not Sakura's own doing that she has it.
The original summoning, yes, but Rider remaining for the subsequent 30 years is entirely Sakura's doing. Not only does Rider remain willingly, but Sakura has the prana to supply her.
Without Zouken setting everything up and finding her a catalyst, she would never summon Medusa.
Only because of a lack of desire. Sakura was fully capable of summoning a servant, she just had no interest in it. And, Rin never found a catalyst either (at least not intentionally), so....
Plus, Zouken doesn't know any of this. All he would know is that Sakura is there and has a servant she can support after the war.