Other than features specific to a given browser—-which seem to be more uncommon as a user than as a developer—-how can browsers lock you in in anywhere near the way a standalone application can?
Yes. As I said, there certainly are APIs which are only supported by a certain browser or family of browsers. I just don’t think avoiding vendor lock-in for browsers is anywhere remotely as difficult as it is with operating systems. Admittedly, I’m a web developer and don’t do native development. However, I frequently run into applications which don’t support macOS or Linux, whereas I very rarely run into sites—especially for everyday tasks—which don’t work on Safari or Firefox.