It means that the outcome of vaccine studies which complete in 2023, e.g. medium-term adverse effects, identification of specific risk pools, mitigations for adverse effects, will primarily benefit those who receive treatment after the studies complete in 2023.

Any beneficial information from a study completing in 2023 would self-evidently not be available in 2021 and 2022, i.e. science would have yielded new scientific knowledge. Participating in an ongoing scientific procedure with partial data is a risk-benefit decision, but that does not make it any less of an experiment, when there is ongoing, parallel research, clinical trials and scientists working diligently to answer open questions.