The matte finish comes from the bead blast preparation. Parkerizing doesn't work on polished parts. Bead blast then a quick hit with steel wool to knock down the sharp points of the surface.

Uh... I worked on volume production of automotive parts, and they were bare smooth steel before the paint process, and very etched dull gray matte surface after phosphoric acid bath before painting. It was a fairly strong acid bath for reduced cycle time, and also to eat off any scale from the welds before paint to have good rust resistance there. No sandblast. In fact, this is why gun barrels going through parkerizing need to first have both ends plugged watertight, to prevent etching the bore, it would have made the whole bore like a very worn barrel with "frosting" just in front of the chamber.

Bluing or black oxide finish, on the other hand, polished parts in come out blued and polished, no serious etch, although they still plug the barrels before bluing.

@Duragrouch, We had parkerizing tanks, hot blue and rust blue tanks in my dad's shop. Every single firearm that got parkerized got bead blasted, and polished areas did not change. Ask any smith and they will tell you the same.
If there was any money in smithing I would have done it my whole life.

I could see bead blasting if you want an especially rough surface after parkerizing, and bead blasting followed by solvent makes for an especially clean surface to make certain the surface gets chemically altered.

The parts we did in production, perhaps were not as aggresively rough as with bead blasting, as after painting, they appeared smooth. But before painting, they went from smooth typical mill finish, to a dull non-reflective surface after phosphoric acid bath. Again, were that not the case, gun barrels would not need to be plugged on both ends before parkerizing, to prevent roughening of the chamber and rifling.

The knife below, previously shiny as new, was treated with a very mild phosphoric acid solution (Ospho), mild enough that a spash on the hand is no problem, and while I wouldn't call it rough, the surface did get etched. If what you said is true, it should still be shiny. The previous production parts I worked on went through a much stronger acid dip, and were more deeply etched and phosphated.

Wouldn’t the acid etch remove any polish and create a profile for the ‘Parkerizing’? Similar to the bead blast, but a finer profile.

That's been my experience, especially using Ospho, still smooth, but removes shine. Stronger phosphoric acid baths will etch deeper and rougher. This is all on plain or carbon steels. Doesn't work on stainless steels, at least not the Ospho; There may be industrial processes that can alter stainless finishes.