re: How do surveyors determine elevation of land?

No, definitely not. The surveys I have done were not based on some cement corner post or anything of the sort. The benchmarks were fence posts, wells, or something else that was not based on anything particular.

Now, the surveying I have done was not civil in nature, but rather agricultural.

Maybe I didn't read into what he was asking as much as you did, because it seemed like a very general question to me, with no indication that he was asking about anything related to sea level (he never said that specifically).

ETA: I don't doubt that what you've said is right, just adding my experience with surveying.