Mio basically was saying that she's kind of glad that Aqua can graduate. Not that she's happy she's graduating, but that through a graduation, everyone gets their chance to say good bye to her and spend time with her before things end. It's not like what happened with Mel where she just suddenly disappeared.
She then talked about how she got along well with Mel, went out with her for escape rooms and other things in their personal time off. But since she usually only uses Discord for things, when Mel's contract ended, she had no way of contacting her anymore.
But since Aqua is graduating, Mio got a chance to appear in her call in stream, offer her a chance for a fortune telling, do another collab with her before she graduates, and after the AkuPeko fortune telling collab, she got a chance to talk with Aqua for like an hour or so. And with that time, she asked for Aqua's LINE contact, so they could stay in touch even after she's gone.
She noted that for most hololive members, she sticks to Discord, and the only peoples LINE contacts she actively mentioned were each of the Gamers, Subaru, and Miko. She might have more but didn't say more than that. And even with the people she listed, she often uses discord anyway.
She also admitted she doesn't use LINE that much in the first place, and is the type of person to read messages but not respond to them right away. She likes Discord because she doesn't have to worry about people feeling like they've been left on read.
One last thing she mentioned about Aqua is that she understands that there are a lot of members who didn't want her to quit, and members who even now tell her that she shouldn't quit and take back her graduation.
Mio, however, feels like Aqua went through great pains to come to this decision, and it wasn't something she decided on a whim, but after careful consideration. Because of that, she wants to respect Aqua's choices.