Bokuto never intended to go easy on Tsukishima because, well, that's how he rolls. Of course, he respects when Tsukki needs time to wind down from stuff (something he should try to incorporate as well), but he never underestimates him.

"I promise on my owl honor, that I shall never go easy on you. In anything. Especially volleyball, eating tons of barbecue and kissing!"

It might sound goofy but- it's Bokuto. He means every single word of the promise he just made.