When you say “true” that cannot mean “materially true” but “symbolically true”, as the Catechism #1383 says “the Christian altar is the symbol of Christ himself”.
This symbolism is implied in what H.U. von Balthasar called Theodramatik, the whole event of divine creation and salvation of the humanity, to which each of us is called to participate as an actor, not as a simple spectator or bystander.