While it is not completely required, it is highly recommended that each user working on a project with cloud based keynotes has a cloud seat. The Revit API allows outside programs to provide keynote data to Revit, but when a model uses this function then Revit will rely fully on that external program to provide that data, which means that without the program, Revit is unable to get keynote data. In this case Revit will act the same as it does when you have text file keynotes and the text file gets moved or changed; the keys will be there but no descriptions will show up, your keynote schedules will have no description text, etc. Because Revit stores the key in parameters on the element there is no permanent damage here; once the connection is re-made all the data will repopulate, but while the connection is broken it will be blank.
So it is certainly possible to have some users without cloud seats and still work on the model. However, their keynote data will be blank so you will need to make sure they do not do any exporting or printing otherwise that keynote data will not be in the export or print. Also, while the data will repopulate for those that have a license, it may be annoying to users to not be able to see the data in their project properly. Because of this we highly recommend a cloud seat per user that is working on the model.