English is studied in primary school, which wasn't the case 20 years ago, let alone 50, so young people all have at least a minimal level whereas most people that are now passing away don't speak any English at all. This alone should be an important factor pushing average proficiency up. The coming of Netflix, HBO and their ilk in the last few years have made viewing TV series in English much more common (regular TV typically only offers dubbed versions). I teach at a university in English, ten years ago we didn't even offer that possibility and now demand is steadily increasing.
OK, I don't have actual data, they do... but given the limitations of their data (acknowledged in the piece itself), I'm taking it with a huge grain of salt.