It is (or can be) just as secure as a non-mnemonic passcode. The mnemonic aspect just helps with typing it out without errors.
You’re not really supposed to remember the mnemonic passcode, but save it in your password manager and/or print it out and store it in a secure location.
Now if you need to use your printed out mnemonic passcode, you just have to type in a bunch of normal words instead of a very long list of random characters and symbols, where it’s easy to make mistakes.