But it still has the same risk we're facing with antibiotics today, development of resistance. It is after all a protein that will target some sort of bacterial antigen

Not necessarily. Its mechanism is very different from how antibiotics work it would seem.

it is that these bacteria are super smart

No, but micro-evolution is fast!

Using a combination of antibiotics to prevent development of resistance.

By your above logic that won't matter, just as combination antibiotic therapies don't stop the development of resistances.

I am not 100% certain how the cationic peptide works, but it would appear somewhat nonspecifically based on surface charge. As such I don't think the bacteria can evolve that away quickly.