If they are ( as you speculate they might be ) "gearing up to do battle with Google and G+..then the battle with G+ is already won..
tumbleweed (the content and "action" in G+ )..as against ( the content and action ) in tumblr..
This might annoy Larry a liitle ..but tumblr was too "edgy" ( with the Pron and all ) to be "buyable" by Google who like to portray themselves as squeeky clean and cuddly enough for kids to take to church..
G will index it..show it in SERPs ..but just like no adsense on anything with "adult or near adult" content..and safesearch being default on Google images in the USA..
Google would not like to own tumblr..at least not in a way that could be traced directly back to them*..
However, as far as a battle with Google goes..Hulu ( a very much USA phenomenon, tiny penetration elsewhere ) ..as against Youtube..a worldwide behemoth..?
Larry won't lose any sleep over Marissa buying that one..He could let Yahoo bleed red ink after the purchase and then buy Yahoo and all it owns in 3 to 5 years from now ( maybe the FTC and even the EU would let him by then ) from a single quarter of net profits..
*Marrissa buying tumblr may actually be a way for Larry to get it ( at arms length ..and after a short wait until he can get it with Yahoo, or Marrissa brings it all home and into "the fold" )..but then tumblr could empty as fast as flickr is doing..