Goku's statements on Buu's power are unreliable because
Buu's power is literally undeterminable, even to himself as Toriyama has stated.
- This is why Goku first said he could never beat Buu
- And then next he's uncertain about whether he could beat Buu
- And then finally he flat out says he would've beaten Buu
These are all contradictory statements that make zero sense unless you factor in that Buu's power is a mystery to everyone, and there are many more examples of characters either underestimating or greatly misreading Buu's power, like Gotenks thinking he could stomp Buu only to be stomped himself.
It is also repeated with Kid Buu, where Goku and Vegeta initially treat him like a joke and that they can take care of him no problem, but then what comes minutes later? Vegeta and Goku both agree Kid Buu could kill Vegeta easily. Vegeta is amazed that SSJ3 Goku can even fight Kid Buu. Goku is unable to beat him and is literally being toyed with, finally needing a Super Spirit Bomb with the ki of Mystic Gohan, Goten, Trunks, Piccolo, everyone else on Earth, everyone on Namek, people in the afterlife etc to actually beat him.
Also, I'm not sure what you mean by "let alone Mystic Gohan" considering Buutenks was stomping Gohan.