Would Mattis actually do anything with this?
The "deployability" standard is a load of shit, and Mattis knows that. And as an experienced commander he should know that there is nothing worse for combat readiness than letting a decent sized chunk of the armed services know that they might abruptly be torn from their units and discharged without honors for something entirely out of their control.
If the White House actually implements the anti-trans "guidance", it'll be a real moment of truth for Mattis to see whether he goes through with actually tearing trained personnel out of their units on the eve of a major troop deployment to a war zone.