A 12 year old does not look or sound like a 19 year old.
This is almost always true, however amongst the thousands of children I've taught I've known an 11 YO who could easily have passed for 20 and I'm currently teaching an 11 YO who looks about 7. There is huge variation in the way children develop- girls more so as sometimes puberty can kick in very young.
That is not me minimising this case. I would assume the judge/jury had good reason to think he was (at least) a sleazy fucker who should have asked more questions. The unpleasant text and sharing her photo suggests he didn't care to find out whether she was under age. It's not a high threshold to say you should be very sure you aren't committing statutory rape before you have a one night stand.
The comments would have been totally different if it had been a 12 yr old boy.
Sadly, I imagine some of them would be about how lucky the 12YO boy was. Some people are dickheads.