He doesn't just deny them, he strongly denies them. So, either someone at ESPN Brazil made these quotes up or Barbosa said them, regrets them, and is trying to do some damage control. It wouldn't be the first time that ESPN has made things up. And, it wouldn't be the first time that a player was misquoted or said something he later regretted saying.
As for Barbosa, several times he has tweeted how much he likes being a Celtic. It just doesn't fit that he would be that unhappy to be asking for a trade or release. When he signed, he already knew about Rondo, Terry, Bradley and Lee and so I don't see him souring so quickly.
I don't speak Portuguese, but if someone does, Gustavo Arruda on Celtics Brazil has the recording of Barbosa's interview and from the rough translation I got from Google, it seems that he agrees that Barbosa is wanting a trade. You can listen to the interview on Celtics Brazil and read the transcript and make your own conclusions. Where there is smoke there is fire. I'm just not sure which direction the smoke is blowing.