It's fine if someone does not believe a rumour, whether it is coming from a forum member, a journalist or the Pope.
It is not fine to call out people for posting info without source or constantly and annoyingly insist on "only official announcements are true".
Certainly not, you don't need an official and public announcement from the team or rider themselves to know certain news are truth. Of course sometimes rumours are only that, and even some deals even fall through despite being verbally agreed (or even signed, as Carlos Rodriguez, in which INEOS had to pay a lofty price to release him from the Movistar agreement), but people should be allowed to judge for themselves which information believe as certain and which not.
By the way, Rui Costa was confirmed yesterday at EF by Daniel Benson, which is as official as you can get without any team or rider involvement on the news.