All agreements are binding, term sheets as well if they are formulated as agreements. But what you agree upon in a termsheet is formulated as "if we do a deal, these are the terms". According to the story, in this case there was also a substantial break-up fee written into the agreement. It might be the case that the signature is all that is missing to make it enforcable.
However, there is without a doubt more to the story than revealed in the article. The missing signature might as well be a red herring, but not because "term sheets aren't binding".
