Science doesn’t not have to be positivist. I think most scientists actually understand that. For example the laws of thermodynamics break down at a quantum level and we’re still trying to come up with and test better models that can incorporate that new information.

What I mean to say about the laws of thermodynamics is they are incredibly useful in describing how and why things change. These were not all worked out when Marx was developing his theories. Yes, Marx and Engels were up to date with the science of their time and they make reference to work and power. However they lacked an understanding of entropy if only because scientists had only begun to experiment with the concept. That’s very clear especially if you read Engels’s Dialectic of Nature. In it he explicitly argues against ideas that would come to be core to the science of thermodynamics.

That’s all to say I suspect if those developments in physics had occurred maybe 40 years earlier, Marx would have formulated a much more precise concept of value. Then maybe he wouldn’t have needed to write so much about linen coats.