Response from support:
Unfortunately it is a hard no for core merchants that are attempting to install Authorize.net in any capacity. I know this is not the news you wanted to hear and I really hope you and your client can find a really positive way forward.
It might be a good idea to let them know about the benefits of Shopify Payments. There are no transaction fees, they have access to Shop Pay and Shop Pay Installments for example and a whole host of other features with proven benefits for eCommerce stores.
So yes, they are trying to push their own gateway.