The Tesla Battery Day holds multiple expectations, but few certainties, if any. We bet on some, such as the "biscuit tin" cell, a much larger unit that promises to come with groundbreaking tech to make it a "million-mile" battery. Another bet is that Tesla will do this battery independently, which would be the most recent example of something Reuters named the "Musk Method." It implies learning something from a partner to do it on its own afterward.
It is worth having a look at the entire Reuters article. Still, a former Tesla executive summarizes the idea when he mentions that Elon Musk believes he can do anything "better, faster, and cheaper." That matches something Sandy Munro already said Tesla does: intense verticalization.