We have three pieces of evidence here:
1)
@BT9494 is our resident expert and claims that it's standard for guilds to have these accounts.
2) Line is NOT widely used in the United States. I literally had not heard of it until tonight and there are many other common services like Discord and WhatsApp for group messaging.
3) Phil used his Twitter/discord name. We know from the leaked usernames and passwords that he only uses "TheyCallMeDSP" or "DSPGaming" for public appearances - for things like utilities or porn he used innocuous names like a normie.
Individually I agree these are not smoking guns but taken together there really isn't another explanation. I could have seen it being for mods but we know he has Discord for that. This is an extreme stretch but the only other explanation would be it's some kind of secret messaging group for his whales and I can't imagine him letting them talk to each other - he is smart enough to know that keeping them in the dark makes them easier to exploit.