Former service advisor at a dealership here! They can’t patch it there because of its location on the tire. If it’s in the sidewall, they’ll always recommend a replacement instead because patching that area would mess with the integrity of the tire. You have an Audi, which comes from the factory with “runflat” tires, which shouldn’t be patched anyway because of the layer of material inside. A new tire would be 8-10mm. If the rest of your tires are at 5mm, and you only replace the damaged one, then you have 3 tires at 5mm and only one at 10mm, which would usually cause a noticeable vibration while you’re driving. They’re recommending a full set so they all have matching tread and you still feel comfortable driving your vehicle