I tend to see it as Mary really thinking Henry did help improve Sotherton by his suggestions, etc. IMO Henry was describing the place to make it palatable to Mary, should she accept Edmund. Thorton Lacy could be a place. etc. But different tones can be read in.

It seemed to me that the lines by Henry after Mary's speech were apologetic... not to Mary, but to Fanny's disapproval:
With something of consciousness he shook his head at his sister, and laughingly replied, “I cannot say there was much done at Sotherton; but it was a hot day, and we were all walking after each other, and bewildered.” As soon as a general buzz gave him shelter, he added, in a low voice, directed solely at Fanny, “I should be sorry to have my powers of planning judged of by the day at Sotherton. I see things very differently now. Do not think of me as I appeared then.”
Henry was aware that Fanny observed more than Mary did on "planning" in the wilderness.