He said in interviews that he was pretending to be a right wing nut job. I don't think that constitutes a valid legal defense of any kind to a charge of creating a public disturbance by inciting a mob. However, I don't think you understand why he was charged. It's not about politics, it's about winning in Court. Prosecutors were pissed when they heard him giving interviews to the press that he should be giving to them. He will likely plea bargain away for favorable testimony against the others. The prosecutors only want to win and the charges were leverage to present a winning case. They would have charged any knucklehead in these circumstances regardless of political beliefs because: (a) he broke the law, (b) he is a key witness, and (c) he was giving his eyewitness statements to the press when he should be talking to LE and the prosecutors.
In his interviews, Sullivan said he was initially treated hostilely by his fellow mob members who chanted "Antifa" and "BLM". This due to him being black. He then started chanting right wing slogans and they allowed him to join. He will likely argue he needed to do so for his own self protection, but since he inserted himself into this mess and became an active part of it, it will not fly, and he will have to bend over, spread his butt cheeks and do what prosecutors ask to get rid of the charges.