This seems like an awful long time to wait to backpedal on an issue that they strenuously claimed was true. Or maybe the headline is misleading.
We don't know how long their investigation took and they are doing a lot more to correct the issue than most online news sources do/have. Most hide it away in the footer, here they put it at the very top where you couldn't miss it and even pulled down the opinion piece. Unless it comes out they sat on this information for a long period of time I think the the Guardian handled this very well (Obviously the best outcome would have been not to write the articles in the first place but we all make mistakes, correcting them is what matters and they did that).
Smells of a Whisper legal department gone on overdrive in the litigious UK courts. At the end of the day, it doesn't really change much of the major findings that The Guardian reported on.