You are exporting a password protected key file from MultiBit classic... This actually encrypts the contents of the file, making it useless for what you're trying to achieve. It probably looks like this:
You need to enter your wallet password at the top, and then leave the password field at the bottom empty... Select the "do not password protect export file" option:
Note: you don't want to copy the datetime stamp at the end of the private keys when trying to import them into Bitcoin ABC...
This will create a plaintext version of the export file which you will find then contains the private keys in the correct "WIF" format... Ie. You'll see keys beginning with a "K" or "L"... Like this: