From my point of view I think he has a case. Disclaimer: I do not own the 2021 DHC, I ordered a 2023 DHC.
From Stellantis official press release here:
For Muscle Car People with Families, the 2021 Dodge Durango Features New Aggressive Exterior Styling, New Interior with Driver-Centric Cockpit and New 710-horsepower SRT Hellcat
www.media.stellantis.com
It clearly stated that "
Dodge will build the Durango SRT Hellcat for the 2021 model year only". So the subject of this matter is "Durango SRT Hellcat", no prefix modifier like "2021". Therefore this sentence means there should not be any other model year, except 2021.