Thanks everyone!
For context, this is my first deal and this guy owns 100s of units - So I do believe that he thinks he can just push me around a bit, and get away with shotty work. This has been a pattern in a lot of the work he has done - including replicating the painting done downstairs upstairs, where it got pretty contentious and the seller's agent ended up agreeing to just pay for it out of pocket.
That being said, the floor marks you see above are just in one room, the master bed. I have decided to just bite the bullet and concede here, as it's not worth holding the deal up, at least to me, for one room (it doesnt help that the seller is overseas until the night before closing..) - we are too close to the close (next wed) and I have reasons for why I'd to stick to that date more or less. The back and forth about one room to be sanded isn't worth it at this point, especially with tempers running high, I'd rather it not come to them saying accept it as it stands or walk.
Lesson learned, check the progress and quality of their work early on and have them provide documentation - not be so trusting that the owner/contractor will get it done properly, in good faith.