[UnFleshedOne]

Without event horizon all the matter that spirals into it would emit a lot of EM radiation as it collides with the matter already there, converting kinetic energy into radiation. In this case they see matter spiraling in, but they don't see results of collision. So they conclude that no photons escape, and that's pretty much definition of event horizon.

[benwan]

No, if it was an "almost black hole" as predicted by general relativity, they wouldn't see EM radiation, due to gravitational time dilation. The "almost black hole" might emit a single photon only once every 100 years in our time.