[benwan]

That makes the point you're replying to. "The most straightforward explanation for our observations is that these objects have event horizons and, therefore, are black holes." No, another even more straightforward explanation is that it's just an extremely dense object that's not a black hole. Even in Einstein's theory, an "almost black hole" suffices. Nothing mentioned in the article favors the former over the latter.

[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.