Moses was a mediator between God and man.
So they took up a position farther away and said to Moses, “You speak to us, and we will listen; but do not let God speak to us, or we shall die.” Moses answered the people, “Do not be afraid, for God has come only to test you and put the fear of him upon you so you do not sin.” So the people remained at a distance, while Moses approached the dark cloud where God was. The LORD said to Moses: This is what you will say to the Israelites: (Exodus 20:18-22)
Moses goes up and down the mountain at least five times that I could count, relaying messages between God and man, discussing it with each party, getting them to understand each other’s position, concerns and responsibilities under the covenant which is being proposed - mediating a covenant.
And that’s the point in Hebrews. Christ is the mediator of a covenant. That’s the whole point of Christ’s mediation - covenant. Christ can be the only one, perfect mediator of a covenant between God and man because only Christ is both God and man.
Whenever Scripture speaks of Christ’s mediation, it is always in the context of the covenant. That’s what Paul was talking about in 1 Timothy - the mediation of a covenant. Jews were covenant people. The covenant was everything to a Jew.
But Moses was also a mediator. The section heading above Exodus 20:18 in the NAB says “Moses accepted as mediator.” Moses mediated between God and man.