The fact that something is ludicrously expensive today when manufactured in ultra-small quantities says nothing about the cost if humanity goes all in on mass producing it.
The only really conceivable shortcut for antimatter would be the discovery of large quantities of it somewhere close enough for us to essentially mine it. We've looked for the gamma signature of matter-antimatter annihilation in space, and we don't see it anywhere.
So it isn't IMPOSSIBLE for antimatter to become affordable, but it's extraordinarily unlikely. If we want some sort of superior rocket, it's probably going to have to involve either some unforeseen tech, or our ability to spend planetary energy budgets on rocket fuel.
And assuming the laws of energy conservation holds up, you need to find a better way to generate the electricity that is being used for antimatter production.
The second issue is less obvious - the Milky Way is some 13 billion years old, has hundreds of millions of stars, but is only 100,000 light years across. Fact - the technological limitations of "today" (this century, this mya), are with empirical certainty not relevant.
Interstellar voyage is not amenable to the time preferences and capabilities of the great ape. The only stuff that could transport life across the galaxy does so slowly and without propulsion.
It says that a Rand study says they might be able to bring down the cost to "$6.4x10^10 per gram" with a dedicated factory. That is still so many orders of magnitude from anything financially feasible.