How? As far as I am aware, you can link an email to an account once it is created, though it doesn't really do much, but you require a phone number to create one. Indeed their website claims you need a phone number or Facebook account. I just tried registering using the Chrome extension and it required a phone number.

https://linecorp.com/en/security/article/164

It appears you are correct. Now I wonder if that's changed recently or age is catching up with me. I've not used it in over a year, but what you said is definitely the case now.