Classic case of a problem that could (and should) be addressed by a conventional database. The only reason to use blockchain is where two or more parties who don't trust each other need to agree on a version of the truth. Traditionally this problem is solved by using a trusted intermediary such as a bank or land registry. With the blockchain the issue is resolved by having multiple nodes a majority of which must agree to validate a transaction AND where no single party controls a majority of the voting nodes.
In this case all the nodes belong to Microsoft and the blockchain is simply being used as a database to allow developers to view their royalties and how they were calculated. Should've been a normal database.
On the issue of GDPR it is prefectly valid to store data that is encrypted with someone's public key within the blockchain so that, in this case, a developer could only view the details of their own royalties. Transparency in the context of the blockchain is the ability for all nodes to view and agree on the integrity of the chain. It does not mean revealing private data that is not actually part of the chain's proof of integrity.