It wouldn't let me add text on initial post - there are lots of evenly spaced waves in newly refinished red oak, picture 1 is brand new flooring, rest is ~28 year old red oak. The contractor has one more coat of poly to do, will that fix this or do we need to sand down completely and start over?

I had this happen at one house I did, though it wasn't this pervasive. Following to see what the floor refinishing guys say. Hope you get your answer.

We call those "chatter marks" from a faulty sander. He probably could have buffed them out before the first coat but now it would be a re-sand

He will need to do a slight crosscut on the first pass, preferably with a different big machine. Chatter can be buffed out with a hardplate but finishers generally use multi-disc sanders these days to achieve a flat floor.

He sounds like an honest guy, since he feels bad for what happened. It's good that you've got someone who's easy to work with like this. I'll be looking forward to the "after" pictures.