You could also see this if you viewed source when it was fully working, of course. And occasionally before images loaded in if you were on a slow connection. I don't think it's any great secret, but still interesting to see.
I was thinking the same thing. I've noticed this multiple times in the past, but I guess alt-tags are too archaic for Average Joe.
I mean, journalists could also see this if they employed people with disabilities that required them to use such accessibility features. Considering it took an outage to show them I'm guessing few to zero companies do.
It happens sometimes in Instagram without any big outage