I think you're right. His appointed term is expiring.
Not being re-appointed is not a firing.
For what it’s worth Dept.........
snip
From the Washington Post article
.....
As is customary, Schwartz, like other presidential appointees, submitted a letter of
resignation to give the new administration a clean start. He had done so before, but
the Obama administration twice declined to accept.
The Trump transition team made the decision to accept Schwartzs resignation,
according to a military official with knowledge of the situation.
end snip