That’s where you’re wrong! If you’re only focused on the story, it’s pretty linear, but since it’s the first generation to make HMs optional (except Cut one time, and Fly for convenience’s sake), you’re going to want to explore once you get those HMs because they open alternate passageways to new extensions of old routes. There’s a side route off Route 1 that requires Surf and loops around Accumula Town to a strength Boulder that connects to Route 2 off the top of my head.
So story is linear, exploration is not. I love Gen 5