>>724585893 (OP)
It makes more sense in the wider context.
Zerg infection has no medical cure. Even the UED, which has tech far beyond the Terrans in Korpulu, couldn't even start to find a cure for it. Hanson is very clearly in a losing situation; she's a fringe world doctor cut off from proper medical equipment and other experts, at the head of a colossal exodus of refugees who are being actively subverted and infiltrated by Zerg forces. This isn't to say that you should write her off immediately, as she might make SOME progress researching the infested, but it's virtually impossible for her to find an actual cure.
With Selendis you're not permitting them to do the wide scale Purification, you're taking the matter into your own hands to clean out the infested settlements to try and save the ones still clean. You'd need to wipe some out, but SHOULD be able to save some, and those you save will be better off for it.
It's weighing thoeries and potential against how hard a proven fact is.