Mexico doesn’t want a Third Country agreement on asylum. The Mexican media has been demonizing Yanquis and the politicians say they will never do it to spite El Norte.
What they want is an unofficial agreement at best that will be disregarded and considered null and void as soon as Trump leaves the White House.
If Trump was really serious he would direct the Treasury to start taxing remittances, which he can do unilaterally.
Under the ED he can impose tariffs unilaterally too.