Your diagram implies that the striker pole slides up and down inside the beam like an arrow on a bow. It's not, it's fixed by a bolt on top and a bolt at the bottom of the beam. So when the mast presses down it pushes the beam down. The tension on the dolphin striker band comes from to places: the beam flattening and the pole pulling the band down because the distance between the beam and the band is fixed. hence everything is locked in place with a minimum of movement.

Expressing it differently the mast pushes down and something is pushing up with exactly the same force to compensate. This thing pushing up is divided between the top of the beam and the bottom of the dolphin striker.

With a compression tube the pressure on the top of the beam is divided by two as it presses both at the top and at the bottom of the tube. Without it the risk to collapse the beam is not negligible. And that's why the compression tube is a big hunk of stainless instead of a flimsy alu tube which would be sufficient to just support the two nuts.