You’re not wrong. This post wasn’t really meant literally.

Speaking in the language of the post:

Well, look. Let’s put to the side whether or not sin^2x + cos^2x is actually 1 or not. In today’s culture, the obvious and natural interpretation of what you said is that sin^2x—cos^2x = 0. But that’s a damaging belief for the future engineers of America to have, that could seriously harm their faith in the math education they received at our upstanding public schools, and so it’s irresponsible for you to go around saying “sin^2x + cos^2x = 1” without clarifying exactly what you do or don’t mean.

Indeed! The post would be boring if none of the bullet points were legit.