I understand your confusion. This genus, specially as it applies to the forms commonly referred to as London Pride (section Gymnopera), seems to be taxonomically garbled. If the plant you have is Saxifraga umbrosa var. primuloides, then it is one of the hardiest of the genus and should survive and flourish in your zone 5 garden. If it is a form of Saxifraga x urbium (the 'true' London Pride), a hybrid between umbrosa and spathularis, then it is most likely borderline hardy.

Regardless, I'd give it a try. This genus provides some of my favorite groundcovers - they spread easily but not overly aggressively, they are evergreen, relatively drought tolerant and will take part shade conditions and they offer a pretty impressive flower display in late spring. What's not to like?