I don't think Buffett himself chose these investments, so it's just namebaiting to mention him in the headline. Since these plants run at a loss (coal power in the US is notoriously lossy), I'd say they're either part of a larger energy portfolio or they're a strategic investment in anticipation of dropping in a modular nuclear reactor. There's quite a compelling business case for converting older coal-fired stations this way, as most of the infrastructure (steam turbines, transformer yards, HV transmission lines) is already in place.
If only the world’s richest man (sorry maybe top three to five I lose track) could afford to clean them up a bit. It’s actually not hard to add catalytic converters. Costs a bit but isn’t hard.