alt Hacker News"False" isn't correct in strict boolean terms either, since that implies that the inverse is true. Claiming "there is extraterrestrial life in the universe" is false is logically equivalent to claiming that "no extraterrestrial life exists anywhere in the universe" is true.
Both statements would have to be interpreted as "false" under your criteria, as neither has any evidence to substantiate it. That leads us to a logical contradiction in which a proposition and its inverse are both regarded as false.
If the statement is being interpreted as "it has been proven that extraterrestrial life exists somewhere in the universe", then it's acceptable to say this statement is false, but making evaluations that depend on an implicit qualifier isn't usually a good approach.
If we strictly follow logic, then nobody and nothing can claim that anything is true or false. We just stick these labels to things which seems to have high enough probability. The problem is that “high enough” is very-very-very different for different people, topics, and even time.