Initial Tao didn't one shot Goku. Akkuman was confident against an opponent he saw one shotting Mummy, who was at least as strong as Pre Water Goku. Therefore, Akkuman wins.
Tao killed Blue with his tongue (who was at least top tier 21st level), and while he didn't oneshot Goku he was stomping Goku with his hand behind his back and nearly knocked him out with a light headbutt. Tao even holding back was clearly presented as being in another dimension compared to everyone else at that point.
Akkuman's confidence could be just brushed off as arrogance, especially since Goku was beating him even before going all out despite there being no statement of Goku changing his strength level between one shotting the Mummy and fighting Akkuman. Or alternatively, you could chalk up Akkuman's confidence being due to his pitchfork (which Goku did struggle against) and his techniques, specifically the Devilmite Beam.