For a studio that's largely been focused on no more than one movie a year, seeing five movies come out in three years is a lot. However, if that's what's on the schedule then it almost certainly means that Pixar has the projects in the pipeline to support it and the studio is confident that won't change. When Mark Nielsen says that all the movies after Toy Story 4 are original concepts, he must be talking, at the very least, about the five films that are currently scheduled for release. He could also be aware of projects beyond that depending on how confident Pixar is regarding which projects will move forward.