and yeah it's true, and I'm sure some of them don't know. But doing that is a learned skill, and a very hard one to master. It's a big reason Science Communication and Technical Writing are seen as cornerstones in their fields and industries.
Okay, so... when people say "write for a high school audience" when they give writing/speaking advice, they kinda don't mean a high school reading level imo
(though, with the literacy rates, it couldn't hurt)
instead, imo the core advice is closer to "outside of whatever they specialized into or sought out intentionally after, people's base facts are learned in highschool, and only made accurate through exposure to information they think is from reputable sources"
or put another way, the further away something is from someone's job or their hobby/activism/etc area of interest, most people's understanding of a thing will be what they were taught in high school. Anything past that takes building up to, if you want to be able to assume the majority of your readers will understand what you're saying.
The flip side of this is that the more you know about something, the harder it is to remember what you needed to know when you were in the position of someone not-even-learning. The Curse of Knowledge.
that's why it's good to keep all this in mind. If you're thinking about it as you're going along, it's easier to know where you need to start and what needs to be broken down vs just discarded, etc.