To a certain extent, I actually agree with this. Although I'd still argue that when the hobby results in you spending over 40k over the course of a few years, especially when you have/had a bunch of other debt and other serious things to spend the money on, you're making a bad decision. Whether that hobby is online gaming or buying cars or whatever else.
The reason I don't think it's a valid argument for Phil is, I don't think it actually makes happy. I'm not sure what you mean by stable either in this context, but I'm willing to argue Phil is in no way shape or form stable. He's ranted about the mobile game trap before, and we've seen some of his old guild chats where he was complaining about how much of a money sink the game is. I'm sure Phil gets some very select moments of joy from it when he's chasing that high of getting a rare pull and finally gets it, but that's really not how you should be treating your hobbies.
To use the yacht example you mentioned before, if the person who bought it is constantly taking it out on trips or hosting parties or whatever, they're getting a lot of enjoyment out of it. It would cross into dumb territory if the person who bought it constantly complained about how much they hate the water or they chronically get seasick or something.
I think the reason people tend to refer to Phil's mobile gaming as an addiction as opposed to a hobby is partially based on how little enjoyment Phil gets from the game. It doesn't seem like he really cares about the gameplay. For Phil, the big thing is getting respect/winning. He just wants to see his name at the top of the scoreboard (or at least in the 4th place slot). That's when he gets his happiness hit. But, it doesn't last, and ultimately there's a new event that requires burning even more money just to possibly end up getting a spot on the scoreboard.