You are both right and wrong.
For email delivery to work, DNS wise all that is required is a valid MX record (but an A record would work absent an mx record). That only covers the MTA though.
In order for imap/pop etc to work, the mail client needs to know the appropriate settings. An autodiscover record linked to an appropriately configured web server makes this a llot easier and would appear to be what is requested.
I note that “autodiscover” should be a subdomain (eg autodiscover.example.com) – so if it already exists the cname should be updated or replaced, and this should not affect anything else A cnsme for the domain name would be a very bad idea (ie you dont want a cname for example.com), and would likely break stuff. If you already have a cname for example.com you are likely doing it wrong, and autodiscover.example.com will be ignored.