That is a bogus answer from them. I get that their insert is for OEM so that a person can replace the insert with no problems. But who would provide a housing that would not accept an existing OEM harness, or their own harness? If you bought both from them and they don't fit together, get your money back and go somewhere else. This is approaching fraud. Filing a part will make it unable to be returned. May cause intermittent electrical contact and lighting troubles.
The catalog says nothing about that OEM story for the housing or for the harness. That's all none of my business, hope you can make it work. Good luck.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn