Paul had no idea or concept of the 144,000 as it was not given to God's prophet John until decades later.
Paul understood about a heavenly Jerusalem. And since those in Paul's time, the majority of which would be anointed, he could properly says Jesus was the mediator between God and men (and women).
People need to remember, that something said in the present tense in the Bible, does not always extend to later times.
Paul is the one that teaches about the two covenants as Jeremiah taught about a new covenant.
Why do people get all worked up about whether Jesus is their mediator? A belief or a teaching can't change the reality of whether he is or not. So just study whether he is or not.