Well, I would wank it that since Alternate Biff "knows" he's about to kill Marty, having been warned by 2015 Biff that someone might ask about the Sports Almanac eventually, it makes no difference whether he tells him or not--the same way he callously abuses that timeline's Lorraine without fear of consequences. This Biff has unlimited resources at his disposal and has never failed at getting anything he wanted; why would he think this would be any different?

(Besides, he's already essentially created a world close to that of Watchmen, with its own President Nixon still in power in 1985, but without superheroes to mitigate its Biff-as-Ozymandias.)

Having said that, most of the big mistakes made in the trilogy are a result of the characters' shortsightedness: Why does Marty forget what Doc just told him about what will happen when the DeLorean hits 88 miles per hour as he escapes the Libyans? Why put an unconscious Jennifer in a downtown alleyway in 2015 after bringing her along on the trip? Why race to catch a train when you've put Buford in prison and now have all the time in the world to get back to the future?