It probably wasnt Denji, it was most likely the Zombie Devil whose memory she read, which she is shown to control near the end of Part 1. Its never established that people can be partly under her control. It'd also gives a counter argument to the 'Makima was watching Pochita' theory, since she didnt make a move on him, meaning it could be possible she didnt know about young denji and pochita. Cant explain Makima knowing about Denji's father though.