I rode the sentinelle about a month ago. there are a few steep kms where an attacking rider could get a gap, as it were ... maybe 30 seconds max. The descent is not steep, however.
Looking ahead, I did the étape du tour on Sunday. If it’s hot and a few riders crack there will be carnage. The final climb was endless. 34 km or so with one brief descent in the middle. 3 or 4 places where a serious attack could be launched. The complication is that if there’s a headwind coming down from Val Thorens drafting will discourage any solo escapees.