I read it quickly, but if I got it right, they're suggesting to put root fs outside of the lvm pv...
Yes, the root file system has to be outside of the LVM -- the initrd does not start LVM, so LVM volumes are not available for mounting at that point.
Nope. That's false. I've installed with root _inside_ LVM.
The installer is a little weird and limited (only ext2 and ext3), but the graphical installer does the work.
Regards, Norberto