The article says "installed border checks", not "walled off the entire border". The article gives no quantification given how many checks there actually are. You can only infer that there's at least one for every border.

With EU border it's always clear that you can cross the border if you want to. E.g. Denmark - until recently - checked me every time I drove across the border, but if I wanted to, I could have stopped kilometer off, and just walked. They didn't control foot traffic and the border fence is specifically designed to be easy to cross for humans (but not boars).