No, it's not. The sub is experiencing untold pressure yes, but that pressure is coming at it from all sides. The only thing pressing it into the bottom is ordinary gravity.
If is sitting on a bottom that is not rocky, then that part of the sub is NOT subject to sea pressure, and it will be pressed down by sea pressure.
It is NOT being acted on by sea pressure from all sides, and this is very well known in UT.
Ever try to lift something off the sea floor at depth?
Pretty much "told" in post No. 31.