Yep, this seems like the exact kind of thing companies would enforce with MDM.
I know one of my previous companies would enforce certain settings were changed if you wanted to install the MDM profile on a BYOD phone (like minimum 8 digit PIN), so requiring you to opt out of OpenAI Siri integration seems like a policy MDMs would offer.
(Though I would never install a company’s MDM profile on my personal phone lol - if they want me to be accessible and have work apps, they can provide a separate work phone)