Stower wrote: The video Mr. Red links doesn’t claim that ‘Sumer’ is the earliest English song – it clearly isn’t – but that it’s the earliest surviving song in English with a complete text, which it is.

I'll only add a nitpick: It is probably the oldest complete text and tune if the common date is accepted. But the date is paleographic, and is disputed. I'm not saying it's wrong, but it is not universally agreed.