Smokestack is the names for a chimney. He's saying in the past general smoke in the area means he couldn't see the chimneys or beyond, but now the site is decommissioned (and I assume other heavy industries have gone), the air quality is better and he can see from the rhet.

I suspect rz2k means that when the plant was operating, they couldn’t even see the towers because of the smoke/smog.

Now that it is closed, and there is no smoke - they can see not just the towers, they now see a long way behind them as well.

There's no smoke coming out.

Best guess: you're confused by the word "smokestack". Smokestack is a synonym for chimney. The power station is closed. The chimneys are still there, but there's no smoke coming out of them.