Purely my speculation:
If Assad is dead then others need to keep things as calm as possible so that chemical weapons can be secured in the first instance, then so that each player can be as influential as possible with whoever next makes up the Syrian government.
Also a need to get the Russian troops out of there if need be without any fuss, or at least to secure their Syrian port at Tartus.
http://www.foxnews.com/world/2013/03/28 ... black-sea/
If it is a false rumour then not sure why, can see no obvious advantage for anyone to spread it if Assad then comes out and denies it.
Possibly though there is uncertainty if Assad is alive or dead, or even still in Syria, so other countries are trying to smoke him out?