The fundamental operation of mmap is to add new entries to the page table of a process, and the precise properties of those entries are heavily dependent on what the arguments to mmap are.

When you mmap a regular file, you're essentially adding an entry to the page table that shares the data with the kernel's filesystem cache. I think he was trying to explain the implications of this fact, but doing so in an incredibly garbled manner, and getting his conclusions wrong.

There are performance implications to using mmap (not always good, not always bad), but both sides of the discussion here immediately dug their heels in on their conclusion without anyone trying to do any analysis to see what the actual implications were, and why.