This is a misleading article.
Most people who pay by direct debit pay one twelfth of their annual estimated bill each month. It is obvious, therefore, that if your contract started after the worst of the winter was over, you will build up a surplus in the warmer months. I'm currently £400-odd in credit. But we've still got the rest of the winter and Spring (when it can remain chilly) to go. My £400 will be eradicated and I'll probably be in debt by April/May. Around 16m households pay their energy bills by direct debit so that works out at an average of about £125 each that the energy companies are holding on to. So I imagine my £400 is included in the £2bn the article mentions. They are quite welcome to it because I cannot be bothered to fanny around getting it back only to have to pay it out to the same people over the next two or three months. As the saying goes - "nothing to see here".