Rather quietly, Google has started making its Duo video (and phone) calling service available to more users. Specifically, some users are now able to receive calls from others regardless of whether they have the app installed or not. It is not clear when the change took effect, although it does seem to be active now, and the non-app version includes many of the previously Duo-specific app features, such as knock-knock. Obviously, while the person can receive a call they are unable to make a call through Duo without having the app installed.
This seems to be a case of Google making use of its App Preview Messaging feature which allows Android users to preview a message regardless of whether the person is using the same messaging service (app) as the sender. Needless to say, one of the reasons Google has likely extended this feature to Duo is in the hope of more users seeing the benefits of downloading the app. A point which seems to be further evident by these non-app Duo calls ending with the recipient receiving a prompt asking if they would like to download the Duo app. As well as the option to block the person who just called them using Duo, if they so wish. For those wondering how to tell the difference between whether a recipient can receive a Duo video call or not - the same rules apply to those who have/have not had the app installed in the past. Simply open the app, head to contacts and look at which contacts show a video (or voice) call icon next to their name. While this previously would have meant that person had the app installed, now, it seems, it simply means they are capable of receiving Duo calls.
What is less clear, however, is how these users are selected. On the face of it, having an Android phone seems a given as one of the main requirements. Likewise, it seems likely that the Android phone in question will need to be running a newer version of Android. Add to that, it also seems likely that the user will need to have linked a phone number to their Google account. While these seem probable caveats, there could be others as well.