That's totally correct.
In this case you have no issues with accessing the source system. Here is how it's gonna be for destination :
- Create Private link for your ADF to expressroute connected Vnet/Peered Vnet https://video2.skills-academy.com/en-us/azure/data-factory/data-factory-private-link
- Create linked service(SFTP) with private ip or private DNS name in ADF.
- Create your Data factory pipeline pulling data from synapse and pushing to SFTP.
Please feel free to comment if you need further explainantion.
Please accept as answer if this helps.