There's no indication that "Highlander : The Source" has been "disavowed" by either Gregory Widen (the original creator of Highlander) or Peter S. Davis (who produced all of the Highlander films and the TV series).
David Abramowitz, who wrote the bulk of the Highlander TV series as well as all three of the recent Highlander spinoff films as quoted on Adrian Paul's blog;
"The ending of this film takes Highlander into a completely new
direction. It's really good. I hope that it is the rebirth of the
franchise. We'll see."
Frankly, the closest I could come to finding anyone involved in the film "disavowing it" is a few quotes from Adrian Paul;
"The Source unfortunately was weaker [than Endgame] because there were script problems
that became apparent when we first started shooting."
thought it [the script] was going to get written by David Abramowitz, and it
wasn’t; it was written by somebody else. I think David had some
interesting takes on it. He came in to try and help us fix the
structural problems we were having in the script on the second
I think the end was a little confusing, as to what was going to
happen and what The Source was about. I think that was a problematic
thing we were facing all the way through, and it never really got
resolved. Other than that, I think it was another really good work. I
think it looked really good; it was a huge-looking film for what we
shot it for. So, basically, I think there are some good things in
there, and some stuff that could have been improved.*
which is hardly the same as a "word of god" dismissal of the film as merely a dream.