No. With 9000 8000 and 7000 series there is tension adjustability built into the derailleur. An inline adjuster can’t hurt but the derailleur can be set up and dialed in without one. I don’t know if I’d call it an upsell since installing an adjuster should only be a matter of a few dollars; more likely the mechanic isnt as familair with 9100 FDs as he should be.