I would love to be able to track the internal debate that Nibali may well be engaged in at this point. Having ridden the Giro the last two years with the intention of winning both, and not having ridden the last two Tours with the intention of winning them, he is due to take on the Tour again. But now, with Froome riding the Giro, he could be concerned that not riding the Giro makes him look like he is ducking Froome. In addition, if he should skip the Giro, and should win the Tour, even if Froome is competitive in the Tour, Nibali will once again be subject to people discounting his victory because of the competition not being top-level in their estimation. Perhaps he is able to set this aside and focus on what his goals are despite the choices other riders make, and more power to him if this is the case.
But I suspect that the degree of competitiveness required to succeed at the level Nibali has in his career is going to make him seriously consider riding the Giro as the race in which he would be taking on the strongest Froome of the year. Which is in itself an assumption - clearly of the recent riders who have made serious attempts at the Giro-Tour double, both Quintana and Contador were significantly stronger at the first race; while Froome may have been arguably stronger at the Vuelta than the Tour in 2017, and may plan his preparation to be stronger at the second race again in 2018, he had sufficient previous experience riding a Tour-Vuelta double to know what to expect. He does not have that experience to draw on with the Giro-Tour double. Nibali does, though he did not ride the Tour in 2016 to win, as his goals for the season after riding the Giro that year focused pretty much exclusively on the Olympics.
So, perhaps if we see Nibali ride the Giro in 2018 it means he does not expect Froome to be able to sustain or regain his form for a serious attempt at the Tour. And if Nibali skips the Giro and rides the Tour, perhaps it does not mean the same things about his expectations for Froome at the Tour. Again, I would love to be inside Nibali's head as he thinks through this. And I also wonder if any part of him is considering also riding the Giro-Tour double. Unlikely, I admit, especially with the Worlds being a likely significant target and goal for him this year as well, but doing the Giro and the Tour with Froome would offer the opportunity to assuage both his competitive drive and whatever susceptibility he may have to critiques of the level of competition he has faced in his GT successes.