That wasn't how it had been explained before, and doesn't line up with my experience either.
Previously, I believe a GM had explained that there is no "T6" for games. Everything is a T5 with a tiny chance to be a Jackpot. The prize messaging will be the same in the mini-games for a T5 as it would be for a "Jackpot".
For digging, there is a message "the hinges lock in place" or something to that effect, which denotes a T5. When you get bejeweled chest that "glows" when it is opened - that is a Jackpot.
For DSD - same as digging.
Which game did you play that handed you a chest? Did it differ from the game where you obtained the boots?
I found 3 Hagfish at the same game. One of them was a jackpot prize (VHCW lance) the other two were crap. Same messaging for all three.