Xie, I'm pretty surprised that you'd have this opinion, because of your stance on remaining strictly canon.
Justin told Daphne about the ILU, and in the telling he never appeared like he was making it up. He seemed wistful and lost in the memory. There was never anything pointing to the fact that he he had made it up. That's why Daphne asked if they were boyfriends. At that point, Justin was clueless about Brian's rules, issues, etc. It was all innocent in his eyes.
As for Justin claiming "he never said he loved me", that was in front of his parents. I always took Justin as feeling somewhat lost and embarrassed for that to be happening in front of Brian. Justin wanted to go with him, and at that point, Justin knew enough of Brian to know claiming he had said 'ILU' during the first fuck was not going to get him what he wanted.
That said, I think Cowlip was trying to go along pretty closely with the British version. In that, Brian did say it, but after a few episodes the Brian/Justin relationship started going off in their own direction. The whole 'ILU' comment might have become one of those blips that Cowlip hopes the fans would forget.