To be clear, setting the record_sessions_percent init option officially enables Session Replay collection at the sample rate. The start_session_recording() allows you to force a session recording to take place even if it is not part of the automatic sampling.
I can see how the naming of the init option does not clearly imply that it is also necessary to enable Session Replay in general. So thanks for the feedback. I just added an addendum to the documentation to make this more clear here and will have my team review this for accuracy before publishing.