Normally if you target a big classic 2-3 weeks after a grand tour you don't go back to altitude in those 2-3 weeks. If you have an altitude camp before the grand tour, then do the grand tour without going ridicolously deep, you basically have the best training block possible. Just also with a lot of accumulated fatigue. So these 2 weeks are about freshening up as much as possible whilst maintaining the form with appropriately structured training . Going back to altitude would inhibit this recovery process.
Of course if a rider fought for gc he might have gone too deep to recover properly at all on those 2 weeks.
Especially in remco case , he is 23, had an extremely stressful season and did already 4 altitude camps. Doing another would probably put on jeopardy his lombardia more than if he under-trained at sea level.