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. 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 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.