Oliver Cowdery was expecting “confederate with X”, the normal usage in English, so he accidentally wrote in the printer’s manuscript “with this man” after confederate. Virtually immediately Oliver realized that he had misread the original manuscript (not extant here), which very likely read “who is this man” after confederate. Oliver crossed out the with and then supralinearly wrote who is; the correction is without any change in the level of ink flow.
The Oxford English Dictionary lists some citations of the predicate adjective confederate where there is no explicit reference to being confederate “with someone”; in the following examples (all plural), the understanding is that these individuals are confederate with each other:
In all these citations (original spellings maintained), the meaning is ‘united in a league, alliance, or confederacy’ (the OED definition). Thus the expression “thou art confederate” without any following with-phrase is quite possible.
Another possibility here in Helaman 9:20 is that in the original text there was an indefinite article a before confederate (“thou art a confederate”). Under this reading, the word confederate would be a predicate noun, and the need to follow confederate with a with-phrase would be lessened. But since the earliest extant reading, “thou art confederate”, will work, the critical text will maintain it.
Summary: Accept in Helaman 9:20 the unusual (but not especially difficult) use of confederate as a predicate adjective without any following with-phrase: “thou art confederate” (the corrected reading in 𝓟, here the earliest extant source).