Why does the movie ignore the first four direct-to-video films when Velma doesn't believe in real monsters? Simple, that film takes place when the crew's been broken up for a while (on fairly friendly terms), and are adults. Here they are still teenagers, so such events hadn't happened yet. Given the van in those movies is noticeably different from the classic machine, it even works that a-way.
For those not up on the most current leaks and super fan speculation, this discussion may have a settled answer come the film's release. The source that found out about this movie two days before the trailer lists the next movie they worked on as "Return to Zombie Island". Some are under the belief this movie will be the case that breaks up Mystery Inc before Zombie Island. The previous film Scooby-Doo! and the Gourmet Ghost had a picture of Moonscar Mansion hanging in the background of where the gang was standing. And a leaked scene of this film shows Daphne has a framed photograph of the gang unmasking the Moat Monster from Zombie Island in her room.
However there is a competing theory that the events of Zombie Island already happened but there's either a in-universe reason such as perhaps they repressed the memories or the 13th ghost himself pulled an example of A Wizard Did It and that is his "curse". There's also a stranger easter egg in that in Scooby-Doo! Frankencreepy one of the faces on the Scooby revenge group was designed to look like the background tourist guy who was in Witch's Ghost, Alien Invaders and Cyber Chase. But some of the older fans instead see most of this was standard Hanna-Barbera continuity flux in action. Where the past might always come back to haunt you even if they don't always play the continuity for anything other than Rule of Fun. Frankencreepy also gave this side of the argument help when it's Credits Gag had a nerdy character get arrested for pointing out continuity errors.
No matter which way this film and the next one play out will probably give some kind of settlement to part of this debate going forward for the foreseeable future. The films will have to make a move in one of these two directions to the others opposition.
According to this film's writer here the theory WB works with general Hanna Barbera Rule of Cool and Broad Strokes was seemingly confirmed on this film. Where they viewed Zombie Island as still having happened in their past even if this film was not entirely consistent. However we should note this was what it was like when he was writing this movie, this would have been written over a year ago and he said he hasn't worked on any Scooby material since. So new people or new opinions may be at WB right now that weren't there then.
We can now say Return to Zombie island has thrown a really interesting development to how this conversation was going. If you look at both these films together they both are attempts to weave past Scooby events into the current direction, both have moments that are mirrors of each other including both leave a different supernatural element the possibility of another return and Return to Zombie Island even keeps continuity right from the events of Curse of the 13th ghost. Like or hate it, this sounds like WB actually intends to go somewhere with this in the future. Otherwise that's a lot of set up and coincidences for nothing.
By the original Series demon creations there never really was a probable point in time to pin their original capture down when considered by the townsfolk's origin. However with now the information they were originally from ancient times and were freed once before and slowly captured adds a lot more wiggle room. As such it makes more sense some of them learned to adopt their powers to different technologies over the years. The gimmick they had when Scooby met them no longer has to be what they've always done and becomes what they were capable of then.
Based on Vincent's history of Asmodeus it also seems to confirm when captured in the chest they were pretty much stuck in a spiritual jail. So it also makes sense that some of the magic users were more powerful then Vincent remembered them in 13 Ghosts. They essentially had the same opportunity Asmodeus did and several of them did manage to one up Vincent on their next trip out. At least temporarily.