They do have a fair use policy on the 1GBit uplink. I know of one report[1] of someone using over 250TB per month getting an email telling them to reduce their traffic usage.
The 10GBit uplink is something you need to explicitly request, and presumably it is more limited because if you go through the trouble of requesting it, you likely intend to saturate it fairly consistently, and that server's traffic usage is much more likely to be an outlier.
[1]: https://lowendtalk.com/discussion/180504/hetzner-traffic-use...
