Source down there.

"You’re not allowed to save modern non-gmo seeds either." implies, that there are no non-gmo seeds, that the farmer could sell, which is also a strong statement

No, there definitely are, but most aren't modern. You're allowed to do whatever you want with seeds that aren't covered under IP laws, like heirloom seeds. The problem is that those (by definition) aren't the latest and greatest, so their yields will be lower, they'll be less hardy, etc.

I'm sure there will also be open varieties, but the problem is still that seed saving is difficult and costly, so most farmers will buy seeds. And the people selling those seeds get less money from selling the old seeds. And that's bad, but not a GMO-only thing.

Here's a great guide as to why the whole situation is rather shit (imho, and in their less-humble opinion too): https://seedalliance.org/publications/a-guide-to-seed-intellectual-property-rights/