You can see something, but that doesn’t mean it’s “space” and “planets”.
Look, he’s wrong, and the flat earth conspiracies are stupid. But, it’s not like the flat earth conspiracies can be debunked that easily. They have explanations of what you can see in a telescope.
The real problem is that life requires that life requires a chain of trust. You trust your parents growing up, then your teachers, the media, political leaders, religious leaders, friends, co-workers, whatever. Their knowledge is mostly based on their trusting various people in their lives, and so-on. Sure, I’ve seen images of the earth from space, but I have to trust that that’s what they really are, not elaborate fakes. I’ve never been to South America, but I have to trust that it exists. I have been to Europe, but I wasn’t personally flying the plane, so I have to trust that it wasn’t some elaborate plan to convince me that that continent exists.
A lot of trust in institutions has broken down lately. Sometimes that’s a good thing. If you look at WWI propaganda posters, they seem ridiculous. It’s good that governments can’t so easily convince their people to jump into a war. On the other hand, this is the result. People stop trusting experts, and start trusting random dudes on the Internet who make a good video.