Speed of Gravity

At https://phys.org/print428727744.html … Gravitational waves have allowed astrophysicists to confirm with greater precision what Einstein predicted in his Theory of General Relativity – that gravity does not act instantaneously as Newton thought but propagates at the speed of light. Is this circular reasoning. We are told that until the advent of gravitational wave research we had no way to directly measure the speed of gravity – yet gravitational waves themselves were measured by using the speed of light to actually pinpoint the date of the collision event that got the gravitational waves into motion. However, we are told that the recent neutron star collision was not only just about the waves but also a burst of electro magnetic radiation in the form of gamma rays also occurred and together they allowed the scientists to set bounds on the speed of gravity that is far more stringent than just using gravitational waves alone. Again, is this circular reasoning?