It's pretty misleading if your not familiar with the interface, isn't it?
The plus to me signifies that the song will be added to a playlist and personally, I'd be pretty confused if it doesn't bring up a prompt to choose said playlist. The minus is even more confusing in that context.
It is some kind of what you said. If you press the + button again you get a menu to add the song to a playlist. Another point that I missed is that the "like" button also adds the song to your favorite list.
That's kinda like double-mapping a button on a controller, yeah you get used to it but it just makes it more complicated IMO.
Tidal still has the heart and hides away the option to add to playlist in a hamburger menu, which adds one tap but personally, I use the heart function way more often so I don't really mind.