I suspect the migration voters did so because of this part:

> app running as a Chrome addon

People who don't know what LINE is will think that means it's a web thing (it is running in a browser). LINE does communicate across all the various devices through servers on the Internet, but so do IRC clients, and like you said, there's not currently a web interface, though historically there used to be (source: Wikipedia):

> At one point Line was available as a website (non-browser-app), but that has been discontinued.

I believe your question would do well on [Android Enthusiasts](https://android.stackexchange.com/). At the moment, I don't have a strong opinion one way or the other on whether Super User considers browser apps to be on-topic applications, but if it does, then your question is fine here too because it's about the interaction between that app and something else.

At least [one commenter](https://webapps.stackexchange.com/questions/100139/line-verify-your-identity-an-error-has-occurred-please-try-again-later#comment87222_100139) on Web Apps agrees that migration there wasn't optimal:

> I'm voting to close this question as off-topic because it is not about a web application. It should not have been migrated from Super User.