I just checked and "Limit ad tracking" was indeed turned off (thus enabling the unique identifier). However, I turned it off and reset the ID, reinstalled IG, created a new account and it still showed FB contacts.
If you reinstall your entire device (thus clearing the keychain) you should be able to break the link.
There's also the issue of App Groups, which means that apps from the same developer (Facebook) can share data between them, so if you have Facebook or WhatsApp installed along they can all talk together and figure out what your other accounts are.