What is your resolution on "one moment"? At the second-or-so level, we'd think that they weren't emitted at the same moment, as the gravity waves come from the orbiting stars before they collide with each other, i.e., mostly prior to the collision.

However, relative to a prediction that they should be separated by over a year, they arrived simultaneously.

You might wonder whether perhaps we correlated the gravity waves from one event with the light from another; this is counterindicated by the fact that the gravity waves match theoretical predictions about colliding neutron stars, and the resulting light data we gather matched theories about what colliding neutron stars might look like, an event never before witnessed. The coincidence of these two factors makes it rather likely these events are connected, in another spectacular triumph of theory predicting events that we've never seen before. I don't know that the popular press articles captured or explained this aspect very well, but I think it is part of why the astronomy community got so excited about this. Only part; there's plenty to be excited about in this event! It may well be the most exciting single astronomical event in my lifetime, because there were just so many bits that came together all at once for the first time.

It was one event, a neutron star merger. The gravitational waves were emitted during the inspiral, the light was emitted as the neutron stars smashed into one another and then blew apart in a "kilonova". The two weren't emitted simultaneously, there was a slight delay between the production of gravitational waves and the peak brightness. However, the important aspect is that MOND theories predict not just a slight difference in travel times but literally months or years worth of delay.