No. Reserve officers remain legally "Officers of the United States." They can be, and have been, involuntarily recalled to active duty to be court-martialed for acts done in an inactive status.
I went back and re-read Article 2 and cannot find any subparagraph that would apply to Kerry when he met with the North Vietnamese. Can you please tell me which part of Article 2 states that Kerry would have been subject to the UCMJ when he met with the North Vietnamese?
As much as I dislike Kerry and loathe what he did in meeting with the North Vietnamese, I cannot see where he violated the UCMJ or was even subject to it during the time he met with them.