Hi David,
the link you provided only talks about the integration of the unitprices in a theme that does not support them by default yet,
but not the write access to the ProductVariant fields through the api.
We are also located in Germany ourselves and face this problem every day, especially when bigger clients who want to migrate to shopify from another shop system or need to update their unitprices through an ERP-software.
It is frustrating since the api needs to exist internally (I’m pretty sure the admin-frontend also only triggers an api..) and is just not published or documented.
Calling out to the german partner manager Roman Zenner did not help yet.
The thread was discussed by a lot of people in the german partner slack channel but nothing happened.
Even noticing losing potential plus leads due to that lack of api support did not bother them.
A very sad story. 
we are still hoping for the best.