We don’t yet know what any of the causes were of that accident. Speaking as somebody who flies spamcans and gliders, poor airmanship can hurt you just as badly in either. Single seater gliders are unforgiving of poor handling of the controls, particularly when close to the stall, as you will be when thermalling. Two seat training gliders aren’t quite so bad, but most competition-capable two-seaters are as bad as the singles. But I wouldn’t say that gliders are any worse in that respect than some SEPs.

You have to have your wits about you when thermalling, because you’re likely (if it’s a good thermal and/or you’re flying in a competition) to be sharing your thermal with many other aircraft, in pretty close proximity. Unfortunately we’ve had a few midairs in thermals. But this doesn’t sound like it was a midair.

I could say more, but I’m wary of putting you off by giving you more knowledge than you want right now!