What the OP is asking about is a device used for VM storage, not the primary storage array. That is likely on spinners. So the SLOG will be on faster storage.
The write path is complicated somewhat by the VM storage being there, but if the SSD is under-utilized, it can work.
Also important to note, an SLOG does nothing for async writes. It only helps on sync writes. If you want to know how much it will help, or not, disable sync writes on the pool. If performance shoots up, SLOG can be useful for you.