[Antoninus]

D had to emerge from retirement for a time to straighten it out.

There is no source that says that Diocletian emerged from retirement. In fact, they all say the opposite -- that when the second Tetrarchy was in the process of failing, Maximian attempted to convince Diocletian to emerge from retirement to fix things. Diocletian refused, famously saying, "If you could see the cabbages that I have raised with my own hands, you would know that a return to power is no enticement."

[SunkenCiv]

Yup, he didn’t gird on the armor and ride out, but he did emerge to convince his former junior emperor to not try to return to power. Eventually Maximian plotted revolt against Constantine, then killed himself.

https://www.britannica.com/biography/Maximian