TL;DR: solid core antimatter rockets basically are the same as solid-core nuclear thermal rockets, which is why they have pretty similar performance figures.
Here's a diagram from Antimatter propulsion, status and prospects from waaay back in 1986.
Solid Core Thermal Rocket (SCTR). The SCTR would utilize the antiprotons by stopping all of the annihilation products In a
solid core of high-melting-temperature material such as tungsten. The
core is honeycombed to allow the heat transfer to the propellent.
Much like the solid core nuclear thermal rocket, this rocket design is limited by the material limits of the heat exchanger and rocket nozzle and so on... if it gets too hot, it melts, and the whole lot shoots out of the back in an embarrassing mess. This means that such a rocket is going to have almost the same operating limits as a solid core NTR, including similar thrust and Isp values. Hence its mention in the 1000 seconds, high-thrust section. I'm not sure why other kinds of antimatter propulsion weren't considered, but there you go.
Is this engine type, which inevitably requires an extremely dangerous fuel and has extremely low TRL, considered to have any worthwhile benefit over a nuclear-fission-driven solid-core nuclear rocket? Is there some reason that antimatter would be easier to manage than fission fuel?
With regards to the danger of the system, it was anticipated that a typical mission for such a rocket would use somewhere between a few tens and a few hundred milligrams of antiprotons, giving an equivalent yield to maybe 20kT max. For comparison that's an order of magnitude worse than the N1 explosion. At least there would be basically no fallout, if it happened on the ground.
Assuming that isn't an immediate deal breaker though, consider the following:
- No fallout from accidents (well, there'd be some fission products from split tungsten nuclei, but vastly less than with regular fission, and no neutron activation products which you'll get from fission and most fusion rockets).
- No radioactive exhaust (though an NTR probably shouldn't have this either, if everything is working nicely).
- You could even do staging with these things, without having to deal with a load of massive expensive hot radioactive fuel rods falling back to Earth.
- Core erosion rates will be pretty low (you don't want it to melt) which makes the engine more easily re-useable without the need for decontamination or reprocessing.
- The engine isn't going to be particularly radioactive after use, making it safer to work around, and has almost no decay heat removing additional post-burn cooling hassles.
- No need for used fuel assembly reprocessing facilities.
- No need for spent reactor core storage locations.
That's quite a nice feature list even compared to fusion rockets, given issues like neutron activation.
- Its efficiency is higher than other antimatter rocket designs, because it captures more of the annihilation products in the engine core.
The big problem, to my mind at least, is that unless you have an economical source of antimatter and mature antimatter storage technology, it will just be pointlessly expensive to operate. Maybe the authors of the paper I linked above, and the labellers of your diagram, had unreasonably optimistic expectations of future antimatter production?
Consider that if you've got even as little as microgram levels of antimatter handy the possibilities of things like Antimatter Catalyzed Microfission or other similar tricks (you can probably use small quantities of antimatter to heat up fusion fuels nicely too) that seem far cheaper and more straightforward and might even give you an awful lot more Isp for your buck than any purely antimatter fuelled drive that doesn't use ridiculous amounts of the stuff.