Regardless of how it’s made, the light has to pass through things - the interstellar medium is not a perfect vacuum. Especially not across one hundred million light-years. The speed of light, as in actual light, goes down in any gas or clear solid. The speed of light, as in C, does not. I think what we just learned is that gravity waves don’t care about stray particles.

If the delay happened the other way around it would be mindboggling.