Surely twould be more efficient just to tell Yahoo rather than trying to bounce the message off us.
If the other potential buyers indicate somehow that they would pay more, then the Tumblr board may perceive that the right thing to do is to be unreasonable on price so that Yahoo! walk away. Tumblr may not approach the others directly, but of course can read any correspondence that arrives.
It's a a dangerous course, as if there is only one other buyer then they may counter with a lower offer, and even if there is a genuine auction then there is the risk that market conditions could change.
And of course all this is also a tactic to nudge the Yahoo! board into making the 'right' decision.