Actually this is Hetzner’s second attempt offering Arm-based dedicated servers. They launched the AX10 model[1] in 2015[2]. It seems like they started to use the “AX” prefix for the AMD-based line later.

[1] https://docs.hetzner.com/robot/dedicated-server/general-info...

[2] https://www.heise.de/newsticker/meldung/Guenstige-Rootserver...