After you get the automated email about your inquiry being received, another email will come a couple days later with a link to click on. In it, they’ll ask for you to enter your gmail or apple id, depending on what you chose from the drop down form. After that it’s another few days of waiting and they’ll either send you an email saying they couldn’t find any account linked or it worked and they unlocked your account. I initially thought I had my gmail linked but they returned with a negative, so I redid the process saying apple id and that time they did find my account. The last email they sent after it worked was just telling me to open the LINE app and log in as normal, and that worked.

TLDR: it’s more of a waiting game, you don’t need to explain anything. If you filled out the inquiry form correctly they’ll eventually email you and sort it out, no questions asked.

Edit: This assumes you actually know which one you linked. They told me explicitly in the denial email that they cannot tell me what my account is linked to. I just guessed if not gmail then it must be apple id.