I think that this article's title is pretty misleading. There's no indication at this time that this is going to be a remake of the first movie. It's not impossible. The most recent movies turned out to be set in an alternate universe from the main series, so the idea that they could use that to remake the first Mewtwo movie isn't out of the realm of possibilities, but I think that claiming it is definitely going to be a remake is a case of jumping to conclusions.
People also thought that I Choose You was going to be movie adaptation of the first season and when the teaser trailer for Everyone's Story first showed up with Lugia's song, a lot of fans thought that meant it was going to be a remake of the second movie. Granted, using such a similar title to the first movie's is a bit more evidence than the past two movies had, but this is still just a teaser announcing that a new movie is going to come out next year. It's way to early to say that a remake is in works based on just a teaser trailer.