I don't think the Base only rule is necessarily in effect during the 28th TB. Gohan's secret is already out of the bag, and he doesn't even have to worry about it anymore because he's not in high school anymore. It's a possibility it is, but again, not necessary.
He said he "keeps slacking off" in his training which means he kept doing it but also kept slacking. It is a present form of sentence.
If he had never trained since buu arc the proper tense would be "kept". Or it would not contain either and the verb would not contain +ing (usually +ed in those cases. ).
I interpreted Goku's words as, "If you had kept training consistently, Boo would be no problem for you." In other words, Goten could have potentially been able to easily beat Boo, but he slacked off, so he never got to that point. It doesn't mean Goten isn't nearing that point (at least in Super Saiyan), but he'd get beat for sure.
SS Goten (28th TB, Goku's intention) >>> Good Boo >>/> SS Goten (28th TB) >>> SS Goten (Boo saga)