It does indicate that he uses some sort of senjutsu or something similar to senjutsu. Then in Boruto chapter 25, we saw him even extending his arm and at the same time transforming it into weapons. In my opinion, it does feel that it is some sort of senjutsu. Now, after that, we got a glimpse of his real power.
Kawaki activated his karma seal whose design looked absolutely incredible. Garou tried to use ninjutsu against him but he absorbed it with his Karma seal. Then he approached Garou and placed his hand over his head and then we saw a huge explosion which killed Garou. The area around Kawaki was also destroyed and Konohagakure shinobi were saved by Boruto Uzumaki and his Karma seal. Since Boruto was able to absorb the blast it is clear that it was ninjutsu that Kawaki used.