I've been developing under that understanding since before Fowler-said-so. His take is simply a description of a phenomenon predating the moniker of microservices. SOA with things like CORBA, WSDL, UDDI, Java services in app servers etc. was a take on service oriented architectures that had many problems.
Anyone who has ever developed in a Java codebase with "Service" and then "ServiceImpl"s everywhere can see the lineage of that model. Services were supposed to be the API, and the implementation provided in a separate process container. Microservices signalled a time where SOA without Java as a pre-requisite had been successful in large tech companies. They had reached the point of needing even more granular breakout and a reduction of reliance on Java. HTTP interfaces was an enabler of that. 2010s era microservices people never understood the basics, and many don't even know what they're criticizing.
I think you are confusing limitations of Java at the time with something else. Interfaces everywhere and single implementation classes has nothing at all to do with Microservices or SOA.
Anyone who has ever developed in a Java codebase with "Service" and then "ServiceImpl"s everywhere can see the lineage of that model. Services were supposed to be the API, and the implementation provided in a separate process container. Microservices signalled a time where SOA without Java as a pre-requisite had been successful in large tech companies. They had reached the point of needing even more granular breakout and a reduction of reliance on Java. HTTP interfaces was an enabler of that. 2010s era microservices people never understood the basics, and many don't even know what they're criticizing.