This one really hasn't been thought through by flat earthers.
There's absolutely nothing about a hypothetically flat Earth that would 'create' a horizon.
Horizons are a property of curvature.
As for it 'rising to eye level', they mean that 'as the observer rises, they can see further, and it appears that the limit of what they can see always appears level with their eye.'
This would make sense if the earth were flat. But, unfortunately, it's not true - and the only way to assert that it's true is to refuse to measure it.
Measuring shows that the horizon is below eye level, and increasingly so as observer elevation rises.
Ergo, this is one of the most straightforward ways to figure out whether the Earth is flat or not.