Same here, baffled at that everyone's just taking the boss's judgement as gospel. Provided that he tested his refactor well, as his colleague I would be stoked someone went and cleaned up my code.
It's not just about reducing lines of code and increasing abstraction, which I agree are bad metrics. It's about legibility. He took a block of 140 lines of dense math descriptions of objects, and reduced it to nicely structured semantic code. It's a 100% improvement, if that code has a bug, anyone could read it and find it.
The author said they would later need support for different behaviour that would have made his code more convoluted. Well big deal, it's not like there was a way to do it without violating open/closed anyway. This code is so easy to understand, anyone can read it and go "ah this won't work with the new requirements, let's delete it and start over".
Not saying this code 100% needed to be refactored, but if you're an engineer and you see it and you think you have some spare energy to pretty up a piece of the codebase, and you diligently test and verify your implementation, why not do it?
I wish my team would do this more often.