That's not how L2ARC works. It's not how the ZIL SLOG works, either.

If a read request can be filled by the OS cache, it will be. Then it will be filled by the ARC, if possible. Then it will be filled by the L2ARC, if it exists. Then it will be filled by the on-disk cache, if possible; finally, it will be filled by a read.

An async write will eventually be flushed to a disk write, possibly after seconds of realtime. The ack is sent after the write is complete... which may be while the drive has it in a cache but hasn't actually written it yet.

A sync write will be written to the ZIL SLOG, if it exists, while it is being written to the disk. It will be acknowledged as soon as the ZIL finishes the write. If the SLOG does not exist, the ack comes when the disk reports the write complete.