The hack is possible however, you need to accept a WiFI connection that has been setup specifically to attain your my tesla app username and password. If you never join a fake WiFi connection their is no way to gain your username and password. Best advice is to always join a reliable and valid WiFi hotspot.
The attack they demonstrated was to trick the user into installing a malicious app onto their phone. The wifi connection was just the means of doing that. If you install random apps from unknown sources, you could easily be installing a key logger or similar, which could compromise your car, bank account, stock market account and a whole bunch more.