Theres a difference in doubting someone and calling them a liar, for instance, if Messi said he 'Im the best footballer in the world' i could doubt him, that isnt calling him a liar, its simply doubting something..
I also didnt say everyone else knew about it..
You don't divulge your age in your profile so it is difficult to determine on what level to deal with this but the level of reasoning in your arguments indicates high school and if that is true much of what you have said can be forgiven on that basis alone.
However words have meaning and meaning has consequences. A riposte such as yours in the Fallout Universe could get you shot.
Your subsequent argument is misapplied, as in it you are disputing a matter of degree but that still makes it tantamount to calling the person a liar. (BTW thinking one is the Best Footballer in the World, and actually being that are two different things.)
However in the original statement we are dealing not with a matter of degree but with a statement of fact. "I just found this great thing!" "I doubt that." Look up the term Ipso facto. You are directly impugning his integrity. You are not only disputing his statement of fact but you are implying additional deceit by intimating that, because the information is so common and readily available, that if he really didn't know about it he is stupid as well.
I'm going out on a limb here because the forum is not a place to teach manners, but if you are to continue to be a useful, constructive and productive member of a world community the effort is worth it.