At the time Johnson is alleged to have made the remark, it was probably a good call.
No one wanted to take over from Cameron. May got the job unopposed by virtue of it being a terrible job that no one wanted. Johnson didn’t have the skill set that May had to achieve what she did in the negotiations with the EU.
By 2019 May had done the difficult stuff that Boris doesn’t have the concentration for. The job of being PM was somewhat easier, and by then Johnson was capable of doing it, and he then considered himself suitable for the job.
The fact that he has been proven to be not very good at it is by-the-by.