LIDAR would have similarly been degraded in the foggy conditions that this occurred in. Lasers are light too.
While I do think Tesla holds plenty of responsibility for their intentionally misleading branding in FSD, as well as cost saving measures to not include lidar and/or radar, this particular instance boils down to yet another shitty and irresponsible driver.
You should not be relying on FSD over train tracks. You should not be allowing FSD to be going faster than conditions allow. Dude was tearing down the road in thick fog, way faster than was safe for the conditions.