There are ways around that requirement. The selecting team and team losing the player can and often do work out a trade or cash settlement, which then allows the player to be demoted.
But the Marlins non-tendered all their first basemen (Bour, Dietrich) and haven’t exactly been actively pursuing replacements. As Ockimey was in AAA last year, he’s close to MLB and one of the more prominent 1b candidates left unprotected.
And he checks all the right boxes the Marlins require - meaning he plays for minimum wage...