I'll post this here as link for people commenting:
Was there an actual Diedne covenant at Augsburg? I'm not too familiar with the House. If so, I guess that could be an explanation. That being said, if you want to use that piece of lore in your campaign as an inspiration, given that a number of historians seem to believe the initial source to be anachronistic and hardly credible, my first thought for an alternative explanation is a local faerie re-enacting a (probably deformed) story about roman difficulties in conquering the province of Raetia. I would not place the faerie and associated temple in Augsburg itself - Augsburg was founded as a Roman colony so that would make no sense, but probably in the general area. A genii loci could always be a tool if you have a strictly local divinity that is not mentioned anywhere else, but I like that explanation less than the faerie one given the details of what I can find on wikipedia about Zisa.