western marxism is more of a school of thought, if we want to be kind to it. It transcends geographical borders - there are western marxists in the global south, trotskyists for example.
Losurdo's bottom line was that western marxism is a school of thought that has mostly had theoretical achievements, and eastern marxism is a school of thought that has mostly had practical achievements, because they are the only ones who have seized state power. (I haven't read the book yet either lmao, it's on my list. but this is what people who have read it explain it as)
So he also put the bolsheviks into that eastern marxism school. I'm sure he goes into a lot more details; even that simple differentiation leads to a whole lot of implications. One thing we can infer for example is that eastern marxism (as he put it, not sure I like the name myself lol) has to contend with the realities of running a country, and inherently gets embroiled in the global contradictions of what "running a country in the 21st century" means.
I assume he doesn't mean that eastern marxism has not made theoretical headways of its own, because they certainly have.