Correct. That doesn't make monolithic systems "not distributed".
Secondly, I don't know why you say "distributed systems are an active area of research" and use this as some sort of retort.
If I say "Is a monolithic app running on two separate hosts a distributed system or not", if your answer is "We don't know, it's an active area of research" or "It's not. Only microservices are distributed"
Most of what people call monolithic systems are indeed distributed. There are usually explicit requirements for them to be distributed, so it's not up to the developer.
But ACID databases provide an island of well understood behavior on the hostile area of distributed systems, and most of those programs can do with just an ACID database and no further communication. (Now, whether your database is really ACID is another can of worms.)
Secondly, I don't know why you say "distributed systems are an active area of research" and use this as some sort of retort.
If I say "Is a monolithic app running on two separate hosts a distributed system or not", if your answer is "We don't know, it's an active area of research" or "It's not. Only microservices are distributed"