In the ams emr management guide (https://github.com/awsdocs/amazon-emr-management-guide/blob/master/doc_source/emr-instance-fleet.md) I read:
... the master instance fleet is only a single instance ...
Does this ultimatly mean, I cannot provision an EMR cluster with instances fleet as high available? Or am I missing something here?
Your understanding is correct. Although AWS introduced multiple master nodes for EMR for high availability in 2019, the same can't be said for fleet instances and the official documentation you mentioned above confirms it.