It depends how you're using send mail at the moment. Sendmail can be run from the command line or it can be run as a daemon, the wiki is talking about sendmail running permanently as a daemon. Sendmail (and every other MTA) uses port 25 to communicate, if you have sendmail running it binds to port 25 - bad, as Zimbra (postfix) uses that port so it woon't run.
So, you need to disable sendmail and stop it from starting at boot time (and any other services mentioned in the docs, wiki etc.).
If you use sendmail from the cammand line then you can continue using it that way.