Do most users think that, though? I remember chat apps minimising to the tray bar since MSN Messenger and Skype, possibly earlier, and it's a common feature in most current ones, as well as in other applications.
Now, it's true that some other chat apps display a floating panel to remind you that you're still in voice chat, but Discord's target audience is videogame players that don't want their media covered by such panel anyway, and most other chat apps let you close or disable such panel as well. Game consoles don't do so either.
I agree with the other points in the case, but this UX design being a GDPR issue makes it clear these people are out of touch with the market.