That said, I don't believe it is true....
I have a hard time believing it too, for the following reasons:
1) Mark is no rock n roller anymore (never was really). Don't know about Dylan and his gang, but on the whole, I have a feeling their intake of intoxicating substances is a
tad higher than what Mark and his band are accustomed to.
2) Mark goes first class in everything he does: travel, food, equipment, accommodation, personnel, etc. Don't know how Dylan operates, but I think the way Mark's tours are organized is hard to beat, and there's no way he'd settle for less. Lugging across the UK and Europe in a touring bus (if that's the case) is not an option anymore.
3) Mark has been and always will be the benign dictator of his kingdom. Even with Emmylou onboard, I don't think anyone doubted who was in charge. Think he wants to be subject to His Bobness or even co-rule with him? Have another think.
4) Mark and Bob are no obvious partners. They did those projects yonks ago, but since then, has there been any contact? (Pottel?) With Emmylou there was at least a long-term recording process prior to the tour.
5) Even a double-headliner thing seems a strange option. What kind of material would Mark present now? New? Too early. Older? Just did it last year.
There are probably more arguments, but I really think it is unlikely this is the big news. I'll be happy to be proved wrong though!
Then again: Maybe he thinks "why the heck not? Why not break the routine, get out those leather pants, grow myself a ponytail (the delay!), and go rockin'!"