I believe POTUS did meet Kim in the conference room at the DMZ
Half of the conference room is located in NOKO the other half in SOKO
Regarding the reference to Forbidden City, the DMZ is considered to be "forbidden" to outsiders
What line on the ground in The Forbidden City/China would POTUS not be able to cross?
Mueller's questions for Trump: Read the full list
www.foxnews.com/politics/2018/05/01/muellers-questions-for-trump-read-full-list.html
Q literally said ground.
"POTUS never crossed the line (ground)."
Of course a line to be crossed does not have to be the literal ground.
In this context however, it makes sense that the line was the NOKO/SOKO boarder.
I presumed that the be the case at the time
More so now after seeing Kim bring Moon across the "line" into NOKO at the DMZ
An anon said earlier April 2nd