"The solid propellant booster ensures the optimum shaping of the missile's trajectory in the direction of the target and separates a few seconds after the vertical launch". You read this part, right? Because it's not like it's written in invisible ink or relegated to the bottom of the page with smaller fonts. This is a classic description of what a booster actually does. MDBA calling it "two-stage" is certainly not in the context that we are discussing "two-stage", as in the first stage is actually part of the missile itself; it's clearly not since different boosters are used on the same missile body. Whoever wrote it is simply counting the booster as the first stage. The booster looks like a booster, functions like a booster, is called a booster by MDMA, but is still somehow not a booster to you?