Browser Exploiting – Using SyncManager to keep Service Workers alive forever

I just read this paper: https://www.ndss-symposium.org/wp-content/uploads/2019/02/ndss2019_01B-2_Papadopoulos_paper.pdf

The authors claim to use the SyncManager Interface in order to keep Service Workers alive forever and thus turning the victim’s browser into a slave.

Is that really possible? I thought the sync process was always initiated by the client and there was no way for a server to force synchronization.

Thanks