What’s left off this article is the technicality of what an event horizon is, for there are several types depending on how you look at it. For example, there is the formal event horizon, which is the boundary across which light rays internal to it won’t reach outward but this is nebulous and spread out across space and time. Then there is the apparent horizon, which is the colloquial one people are more familiar with that is the schwarzchild radius static in a moment of time and space that is the boundary where light rays can’t escape from. When two black holes merge, as the two separate horizons approach each other, the localized spacetime can become closed off from the rest of the visible universe without passing either of those two radiuses and before they merge because the average mass in that spacetime vicinity forms a horizon around the two merging black holes.
PBS Spacetime has a nice episode on it. Detailed measurements of mergers will give us a better understanding of the entire picture of how these events play out and shape spacetime.
Still, without being in the gravitational-wave field, it's still pretty hard to see what all the fuss is about.
Ha, just like being close to a supernova can cause such extreme neutrino flux you can actually die of radiation from it interacting with your body, being within a couple of horizon widths of the merger can probably put such excessive stress on your body from the force of sloshing space time as to actually kill you. Which is kind of insane.