Similarly, if I build a microservice architecture, and then say "My monolith is too hard to manage because it's in so many pieces" I think it would be fair to say "You didn't build a monolith".
You're sliding the definition of microservices over from the obvious one (many small services[1]), that's what's a 'No True Scotsman.'
With that said, I don't think you necessarily committed a fallacy, it's just a matter of phrasing. "You weren't doing microservices at all" is the fallacy, but the underlying message is sound: It's not enough to split a monolith into services, you also need to spend some time architecting the services to play well with each other.
But I think it's unhealthy to say "you didn't do multiservices" instead of "this issue isn't inherent to multiservices, you can overcome it," because the former sets up multiservices to be a silver bullet.
[1] We can expand this definition to how Fowler and the like define it and still run into 'split monolith' problems (dependency issues being the biggest in my mind).
It sounds like what they did was attempt to split a monolith into separate services, which is distinctly different from a microservice approach.