The universe is not a hologram stored on a flat plane.
Holograms are not stored in 2 dimensions.
If you project a hologram you have to project it onto something.
In which case I think they're talking about projection from n-dimensions onto m-dimensions, where m < n and n=9 in this case.
It's not a plane per-se. It's a smaller set of dimensions with a representation of something in higher dimensions. Relative to 4 dimensions, 3 dimensions is a 'plane' (I forget the mathematical term).
Physicists need to stop tugging their dicks in mathematical fancy and start developing ways to TEST things.
Well, at a certain point, theoretical physics devolves into abstract math, and people trying to come up with models which explain what we see.
I mostly agree with you, because string theory always seems like it's so abstract as to serve no purpose. However, I'm also not qualified to do the math and fully follow the logic behind it.