About training overhand, reverse, hammer curls for big arms without flexing?
Is it even worth thinking about? Or just broscience?
You posted a video of the biggest bro-science contributors in the YouTube community and ask if what they are saying is bro-science?
Why don't you run a Google search for 'Hammer Curls' and return with what you learned. You will be able to answer your own question whether or not hammer-curls are necessary for arm development.
edit: did not look at the video