I mean, no offense, but it is pretty bizarre, I can just as easily say argue that in his mind knowing that a bug is animal is pretty obvious. I myself have never met anyone who doesn't know that bugs are animals, it just seems like common sense, let alone some obscure fact about biology.
He's not trying to debate you, he's just shocked. Though to be fair, I've noticed that both you and praline are older than most members here, perhaps it is due to how knowledge in biology is a lot more accepted and encouraged nowadays in contrast to back then. Along with improved education.(Though it's still not all that great in the USA at least.)
Never assume common sense is common among everyone in the world. That's a bad way to debate.
If they don't know, explain. If they do know, cool. It's understandable he thought it was common sense.
However, that doesn't make it a constructive point to make either way and still adds nothing of use to the conversation. The point is to always try to help somebody remember or teach them something if they're wrong.
I stopped using common sense as a legit point when I realized that it's actually subjective. What's easy for one person to understand may not for another. People think differently. Could be mental issues, different opinions, etc. I get why he said it. But it still was an issue because it provided nothing constructive.
And yes, you have to remember that education has changed a lot. As you said, some things aren't as emphasized. Hell, I don't remember anything about Biology outside of high school, and about all I remember at best was DNA stuff, and barely. I'm pretty sure Math, Health, Gym, and Social Studies were significantly higher points at where I went to. Another thing is that I went to Catholic Schools. Evolution was definitely not a high priority due to that at the time. Thankfully that's changed a lot, but it wasn't when I went there. I don't remember honestly any Biology stuff during Grade School. Now, it could be because I had zero interest in it, or it was because it wasn't taught properly. I don't remember. But yeah, as you said, different priorities now.