I'd say Devil Man's confidence shows he could take care of someone who could one-shot Mummy, but the power Goku used to one shot Mummy was even beyond Akkuman's estimations, who maybe hadn't a solid grasp on Goku's power due to him barely doing movements or so.
Well, like you said, it was stated Goku would probably not be able to beat Mummy at the 21st Budokai, so I generally place this latter at Goku (vs General Blue level). Yet, Goku is able to oneshot him, when not even going all out. Still, Akkuman is confident he could defeat someone who one shotted Mummy. Still, Goku could oneshot Akkuman at full power, showing post Karin Goku could oneshot someone (Akkuman), who could oneshot someone (Mummy), who could probably beat 21st Budokai Goku. The gaps are really massive.