They say the current extension will only be on the Swiss side, while the line across the border might follow later: https://www.ge.ch/actualite/extension-du...02-2021

Yes, the line will be extended to the ZIPLO at first, in 2022/2023, but works have already begun in St. Julien as well (one street near the train station has been closed and is undergoing widening and other modifications) in order to quickly extend the line to there, which will probably happen in 2024 ;)