Another theory, and I obviously can’t speak on Verizon’s behalf, is that maybe Verizon whitelists phones based on the phone’s compatibility to Verizon’s own apps and settings control mechanisms. As @johnk mentions, the hotspot capability is an optional add-on for a phone plan with Verizon that has an additional cost. If Verizon can’t lock down the phone to disable the hotspot capability for those that don’t pay for this plan add-on, customers would be able to use the feature (as long as the phone provides it but most smartphones nowadays do) without paying Verizon… the Librem 5 and PinePhone are good examples of that since their capabilities cannot be artificially reduced based on some wireless provider gatekeeper’s whims.
Another possibility is that Verizon offers device-specific customer support. I’m sure most people here are familiar with the proficiency level of a lot of the industry’s phone or chat based IT support teams and their limitations. There is probably an internal Verizon database for common issues and solutions for each whitelisted device that their technicians go through… like a checklist. Step 1: reboot the phone. Step 2: if it’s Google Pixel : do this procedure to reset to factory default. Etc.
Although my PinePhone Pro is not denied network access or service in my Verizon plan, it definitely feels like it’s considered a “second-class” citizen.