Interesting - at one point I was told by a Sixt rep, that to get the Diamond level, you need to be friends with Regine (Sixt) ie. it's an invitation only level (and the Sixt UK website still says so)

PS - once you're matched to Sixt Plat, you're that level forever I matched a couple of decades ago to the status on an airline that doesn't exist anymore and the Plat status stuck. Not that it's a great things, but there have been a few nice upgrades along the road. (pun intended)