I've been wondering about it, since it does seem very odd. On the one hand, she told us that there would be:
(the unavailable tweet is from The Grocer, announcing her as their 'Hero of the Year' - it's possible that she's actually intending to say that the cover shoot & interview is for them rather than the guardian, but you'd have to ignore the tweet she's replying to for that to work)
Other points in favour: it ties in with the book launch date - it's the week when you'd expect an article to appear if one was going to; she (or her publishers / publicist) clearly has some remaining relationship with the Guardian, and there might have been some sort of deal done around the cancellation of the Guardian Live event; it's a slow news period, so any content will be welcome.
On the other hand: A cover shoot and multi-page interview?! Really? Monday is a strange day for that, given that the Graun's food supplement is published on Saturdays. The Guardian seem to be a lot less keen on her than they were (and, again, there's all that business around the Guardian Live which looks bad for her), and they must be aware of all the new, more widespread concerns about her behaviour.
Therefore: Who Fucking Knows... but I'm looking forward to finding out.