Duo is Google's video calling app so it does use your phone number as your identifier (and the phone numbers of your recipients). But once you initially establish your Duo app with your phone number, than it's just a matter of using your Google account to log into the app on your tablet (and using the Chrome browser, do the same on a desktop or laptop.)
Side note, there's been a lot of online mentions the past few months about rumors on Google killing off Duo and merging it into its Google Meet service. This could just be idle gossip but it does fit in with Google's pattern of getting us hooked on various services and then just killing them off. I think Duo works really well, it's more of a single purpose app without a lot of bloat and multiple functional additions tacked on to using it.