Not sure, but in theory: when you let the procedure finish (open the link and install the app on some device, logon with the user account), then just remove the app again and let the user logon to the web app from any browser & device.
Correct, just use a mobile temporary:
LogOff, Create a New Athom Account, accept the invite link,
then log off and only use the WebApp.
You can use the mobile again for the original account.