There was, at some point during the WHC proceedings, a "Welcome" offered out to the Singapore delegation (who WERE present!) but I can't remember the exact words used.
Certainly there is nothing on the Web from any Singapore newspaper or UNESCO source indicating that Singapore has yet "ratified" or "acceded to".
Its intended presence is identified in the "list of Participants" but NOT among the group who are "Parties to the WHC" (nor of course those on the committee itself). Instead it is listed in another category :- "Entities maintaining permanent missions to UNESCO". See http://whc.unesco.org/archive/2012/whc12-36com-inf2.pdf
So, it does indeed look as if, for some reason it hasn't formally completed ratification! Unless possibly it did it during the WHC in some ceremony - but, if so, that doesn't seem to have hit the newspapers and, in any case, I don't know if final legal "ratification" is a government action which has to take place in Singapore (or Paris?) or a "ceremonial" act which could have taken place in St Petersburg?