WTF is wrong with just copy/paste the same big unimportant file (like a movie) until you run out of space and then just delete them all? So much unnecessary hurdle with different utilities and commands that made my head spin. What do you think those utilities do in the background anyway if not filling the free space with junk, just like what a copy/paste does?

That won't overwrite pages not allocated to a namespace (which can happen due to wear levelling/underprovisioning, or because the controller has decided to stop using that page because it's unhealthy).

Flash looks like a simple array of blocks, but under the hood there is a controller that allocates writes to different pages. You need to tell the controller to erase all pages if you want to guarantee data destruction.