Dear Pasq,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and to my knowledge, it seems the users should have atleast the permission to add and edit permissions for using @mention feature in the shared Office files.
I agree with you having the ability for using the @mention feature even with reviewer permission will benefit the users and I suggest you to add your valuable idea as feedback in the SharePoint · Community (microsoft.com) which is the best place to share our ideas and improve the Microsoft products.
Appreciate your patience and understanding. Have a good day!!
Best Regards,
Sophia