Why do they say otherwise on their own website?

https://www.amazon.de/b?ie=UTF8&node=17666138031

Well I have bo idea. All I know is when I purchase on either .de, .co.uk or .ie the transaction goes through immediately. I have 3D secure enabled.

The text on that link of yours is sneaky though:

"...you MAY be asked to..." (Emphasis mine)

Because you can add them to a "That Store is safe" list, and you won't get asked again. You probably did this at some point

They recently enabled 3DS for business accounts, on customer ones it’s indeed still disabled