Imo Marxism should be understood as scientific but not a true science.

People get caught up in defending or debunking the idea of Marxism as a science. It's not physics, it never will be and that's okay. Lots of things are scientific without being a science, depending on exactly how you choose to draw those boundaries, and in fact a lot of the soft science is scientific without necessarily being a science. But what soft sciences can yield with regards to information and insight and analysis is still invaluable and irreplaceable, even if it isn't on the same tier as chemistry.

Usually when people try to debunk Marxism as a science they are either paraphrasing the Popperian critique of Marxism or they are regurgitating someone else's paraphrasing of the Popperian critique. If anyone ever starts talking falsifiability in a discussion of Marxism and its scientific approach you can immediately discount this person because this is a trash argument; plenty of things that exist squarely within the domains of hard science are not falsifiable themselves and plenty of things that are a part of science happened outside of the scientific method. So basically by Popper attempting to exclude Marxism (and psychoanalysis) from his definition of science, he made it so narrow as to exclude large swathes of science itself. If someone wants to pick that particular hill to die on then that's their prerogative but doing so makes that person more anti-science than the person who considers Marxism as a true science imo.

But this is all an inherently negative project. Marxism holds a lot of descriptive and predictive power. Like anything to do with people and politics and economics, it's never going to be absolute and it's always going to be messy and whatever you use to analyse this stuff will necessarily be incomplete and imperfect. But the negative project of attempting to disprove Marxism being scientific/a science isn't about improving upon it or contributing to how it analyses the word, instead it's just an attempt to undermine or negate the importance people place on Marxism by gatekeeping and trying to cast it as being undeserving of the prestige that scientific endeavours are given.

If you want to head off a discussion that is going this direction you can foreshadow them being about to discard a large part of the sciences, as mentioned above, or you can ask the person if there are any aspects of Marxism that are scientific to draw out their ignorance on the topic or you can ask them how they think Marxism could become more scientific. This usually stumps the anti-Marxists.

But ultimately Marxism doesn't need to be a true science to be scientific and it doesn't need to be a true science in order to be an extremely valuable way of analysing the world so I generally don't bother with discussions about whether or not Marxism is a science because the majority of people who want a debate on these matters have near-zero awareness of philosophy of science so the discussion amounts to watching someone wade around in the shallow end while they play pretend and convince themselves that they have some epic gotcha when really they don't.

Not trying to imply that this is what you're are doing right here in your post but it's something that's pretty common amongst critics of Marxism. It's a bit like people who argue that Marx's approach was based on an incomplete or a flawed anthropology. That might well be true and I'm very interested in hearing the critiques of Marx's anthropological foundation but just because the starting point was wrong or inaccurate doesn't mean that where it goes and what conclusions Marx draws are therefore equally as flawed or false.

To illustrate this point a little bit, imagine if I encountered a car for the first time in my life and I pulled the engine apart and studied how it functions to conclude that there must be microscopic gremlins that push the pistons which create the force that cause the driveshaft to rotate which transfers the energy to eventually propel the wheels into motion. Obviously the starting point of my analysis is completely incorrect and it's downright unscientific in the most extreme way. But my understanding of the mechanical principles of what happens beyond the cylinders is accurate. This means that we can improve my theory but it doesn't mean that every conclusion I arrive at after the microscopic gremlins is therefore as incorrect as my starting point is.