The point was not the method of destruction but that they were forced to do it at all, on a questionable legal basis, on computers they were still using (not disposing of).
I'd like to underline this. The Guardian co-workers were forced to perform the entire deletion _themselves_ (!!!)
It is almost like a combination of some kind of forced, unpaid employment (akin to "slavery"), shifting the blame ("you did it yourself"), or retribution ("you abused it, you take care of it"). Very twisted.
They were “forced” but not in that sense. The Graun offered to do this instead, and so GCHQ came along to watch. There is a problem but not with forced labour.