Presumably it's built on top of their existing VPS and dedicated server boxes, which AFAIK only have 1gbit uplinks, so they'd only be able to exceed that speed when the bucket is split between multiple boxes.
I am not sure about the exact specifications anymore, but they are building dedicated servers for this with custom chassis where each server has a ton of drives and I think they each had 40 GBits networking. These are special servers that are not available to customers directly.
[1]: https://docs.hetzner.com/cloud/technical-details/faq#what-ki...