BFG-9000 :
How thin can it be if it has an optical drive?
This video shows the standard 2.5" SATA HDD in a X555 and the unfortunately complicated disassembly process to reach it. If you tried to do that in a shop you would likely be escorted out.
The video you linked isnt for the X555BA-DM100T. Its a different version of the X555, different color bezel around the keyboard. I couldnt find any specifics to nail down which model the vid was using, but its not the one the OP listed.
This machine has an optical drive, but looks like it was squeezed into the design. This chassis looks like a wedge compared to other models in the X555 line. Im not finding any variations other than the one with the M.2 slot SSD. It may well be able to fit a standard drive, but Im not finding documentation for it.