I strongly suspect that the restrictions on accounts for residents in Germany are based on EU-wide cross-border VAT regulations: Google wants to avoid being responsible to collect VAT from you that it would have to forward to German tax authorities if you would act as private person. Instead, as a German business, you would be responsible to forward VAT to your local tax authorities directly, with Google not being involved.
The applicable VAT regulations here in case you would be a German business would be EU directives related to VAT reverse charge procedures and they would apply because the Google legal entity with which you would be contracting would be a EU entity, namely Google Ireland Limited, according to the Google Cloud Platform Terms of Service. Practically, the EU VAT reverse charge rules would relieve Google from VAT accounting and administration.
It follows that if you only want to try out the API and no actual payments will be made to Google from you, you would be fine. However, if Google will charge you for API use, they say by this clause that it is your responsibility to report VAT to German authorities. As you couldn't do that if acting as private person, Google would be responsible and could be in trouble if German authorities would investigate. In the end, by this clause they state that you'd be liable towards them if they actually run into such trouble.
So, technically it is not illegal to use the API for non-commercial purposes but you would commit a breach of the agreement between Google and you that could lead, at least in theory, to a liability of you towards Google if German tax authorities would ask Google to pay the VAT you were supposed to pay.