sonofjones said:

I don't see much reason for this since he had underestimated Luke by assuming that pushing him into the carbon freezer would be enough to defeat him when he showed himself, by jumping out in time, to be in tune with his surroundings, which is why during the rest of their duel he didn't hold back as he did before

Darth Vader was toying with Luke up until he struck from the shadows. With more of Luke's skills on display, the "Impressive" line makes more sense.

And don't forget about the "Obi-Wan has taught you well." being more significant if removed from ESB due to Luke's skill coming full circle.