Remember what issue #51 of Superman showed us? It said Clark is dying because a number of reasons and one of those reasons is the cell damage from becoming "the god of strength" via the fire pits. Lex might already know Clark's dying ala All Star Superman. Hell, if that's the case then Lex is part of the reason Clark is dying (just like All Star) since he's the one who suggested the fire pits to power Clark.
Lex could have the suit already locked and loaded by the end of Darkseid war because of this, right?