Xert Segment Hunter (XSH) is dynamic power pacing rather than factoring in slope % changes.
XSH is best for shorter segments. See this related post.
On a long hill climb TT you’re best off maintaining a tight target power range regardless of slope %. There are exceptions, of course, but if you follow Phil Gaimon’s advice for long climb record attempts the first third should feel easier (suppress urge for a hard start), second third is a concerted steady effort, and final third will be hardest hanging on to your watt target to the finish line.
Mentally breaking the effort into thirds (20 min in your case) gives you three milestones to focus on. Power-wise that means as steady a burn as you can manage. 
Here’s Phil G’s power, HR and cadence profiles for his MWARHC record ride on Aug 20, 2022.
I’m sure there is an Alpe d’Huez FIT file you could download from his Strava account and plug that into XFAI Race AI. You’d need to contact him for a date or view the segment on Strava and pick someone with a public profile.
My guess is it’s a similar power chart with short blips at switchbacks which the MWARHC lacks.