Correct. When a sync write comes into ZFS, ZFS won’t reply until the underlying device has committed the write to non-volatile storage - enforced by sending the SCSI SYNCHRONIZE_CACHE command to the SLOG device.
A consumer SSD without PLP - and a volatile write cache - will get this command, and have to actually program the NAND cells with the requested data before responding OK.
An enterprise SSD with PLP knows that its onboard supercapacitors have enough power to flush its volatile cache to NAND, so it will simply reply OK immediately, and begin programming the cells - even if power is cut immediately after the OK is sent, there’s enough power stored in the drive itself to complete the programming process.
The difference between the two can be immense - we’re talking about the difference between “hundreds of megabytes per second” and “single digits, maybe a dozen MB/s” at the smaller record sizes.