Okay, not 121.5 then but the Full Sutton frequency. How would he know its frequency? Gliders don't normally use an info frequency like London info, so they can't ask there. And looking it up on the map, then selecting it, while in a high sink situation and trying to spot traffic at an unfamiliar airfield... Kinda hard to do.
Furthermore, the radios I've seen in the gliders I flew all were of a rather old design, with no standby frequency, and individual pushbuttons for each digit. And due to cockpit layout and the way you sit (lie) in it, they were just not quite within reach. Not an excuse, but possibly a factor.
Now to be honest, I'm not trying to make up excuses for this pilot to land on the wrong runway, unannounced. I think he should have never gotten near or over the ATZ in the first place. So from what I can see his first mistake was losing situational awareness, or maybe even getting lost completely. From that point on, it all went downhill. But one of the factors in that downhill slide is, in my humble opinion, the way the glider training curriculum simply ignores the whole concept of VHF radio until you're well into the x-country stage. And then still radio is virtually only used if a controlled airspace transit is called for, not to do circuit calls at the landing site.