It depends what you mean by "it's impossible" when you are discussing 'good-guy backdoors'. I don't think the article was saying its impossible just because it

It depends what you mean by "it's impossible" when you are discussing 'good-guy backdoors'. I don't think the article was saying its impossible just because it is technologically hard. The article discussed at length that what was impossible is that the keys would be guaranteed to stay in the hands of good guys, and that the good-guys would always be good. I think it is perfectly reasonable to say that that is impossible.