Just to confirm what someone else said and to also chime in, YES, in order to open up the last room to catch the blasted ! and ? UNOWNs, you MUST open up the chamber that requires Ho-Oh.
Why?
Because you need to get access to that 'writing' that is available when you open up the Ho-Oh so you can report it to the researcher who can THEN open up the last room IF you have 26 of the UNOWN.
My guys are at level 30-33 and I'm nowhere near capturing a Ho-Oh so this just stinks.
Clocked in some crazy hours between Pokeathlon, Safari, level grinding, breeding, Voltorb, and who knows what else and was hoping to get the Ruins sidequest done, but nope.
:(