Yes, they have timestamps. But if you do buffer (or better to say, delay it), you introduce latency, which is even more worse then jitter. The ideal is 0 latency. And another downside with buffering, you would need to manifest the buffer time at all device you trigger to be the same time otherwise you do not stay in sync.

Edit: Actually midi note on events that are being sent to devices do _not_ have a timestamp! Only events that are persisted in a file may have timstamps.