Maybe they don't want to have two All Riders movies in a row, and they are just going to incorporate Fourze's aspect of a tribute series into this movie.
Well, Decade's part of Movie Wars was at some point meant to be an All Riders movie, it even got listed as such by some theaters (back when it was meant to be just a double feature, with All Riders vs Decade and Double Episode Zero), so, another All Riders movie in the same year of release of Let's Go Kamen Riders isn't completely out of question, even if it's unlikely.
The difference between these two also would be bigger than the one between All Riders vs Dai-Shocker and Movie Wars 2010, because Let's Go Kamen Riders wasn't the summer movie.
Let's Go Kamen Riders DVD is getting released in October, IIRC. If they only really start advertising this new movie in November, it could be an All Riders, or just feature some previous Riders, without stepping on the toes of Let's Go Kamen Riders at all, due to the front loaded sales of most DVDs in Japan.