Board Thread:Episode Discussions/@comment-25319479-20171206103000/@comment-35622277-20171207220131

Most of Mitsuki's abilities were barely shown including his mastery over them. He seems to be able to enter Sage Mode at will like normal users of it just by gathering nature energy.

It's not solely on Boruto's ninjutsu or taijutsu prowess that puts him at Chunin-level. His strategies and ability to adapt in combat are what makes him Chunin-level. There are also a bunch of fodder Chunin everywhere.

He's just a natural prodigy.