I think you are both correct, but are talking about slightly different things.
The statement is that it can focus on materials up to 0.5”. It does not say that the focal point can move 0.5”—which is what you are claiming they said. So, you are right that the lens travel is restricted.
But there is also the question of depth of field. If the beam is focused sufficiently at 0.5” even though it is .04 to .08 away from optimal, then they are stating the truth, just as much as you are stating truth that the lens travel is only 0.42.
My range of possible errors is based on the supposition that they may have the lowest point also slightly above the bed.