In the most simple scenario where you inject something inert (literally anything for that matter) into the exhaust stream, you increase its momentum but decrease its velocity. That means higher thrust but at the same time lower ISP. For this fusion enhanced engine here they don't seem to get anything from the fusion process energetically at all, which makes the numbers extra weird. We'll have to wait and see if this whole concept even works.
No they aren't. It seems their design only really reduces the amount of unpaired electric charges expelled, which otherwise would get pushed back to the cathode because of the plume's total charge density. They don't even use the energy released by fusion itself, because for that you would need to fuse the particles before they reach the exhaust nozzle (which is super hard and also the reason why noone managed to build a real fusion rocket so far). That's why they call it a "fusion-enhanced" instead of "fusion-powered" engine. They talk about it in more detail in another article:
