Nibali probably thinks Roglic is playing games by not going to the Tour with him.
If he goes to the Vuelta there is actually a bit more timetrialing that in the Tour this year, but the climbs will definitely not suit him, the Tour has pretty regular climbs of up to 10%, meanwhile in Vuelta this year, stage 7 MTF:
Stage 13 MTF:
If he goes to the Vuelta there is actually a bit more timetrialing that in the Tour this year, but the climbs will definitely not suit him, the Tour has pretty regular climbs of up to 10%, meanwhile in Vuelta this year, stage 7 MTF:
Stage 13 MTF: