Bots can't do that. If you had placed a limit a limit order @...

Bots can't do that. If you had placed a limit a limit order @ 22c then the first time a bot could have detected it, it would have already been filled on the ASX @ 22c. The reason it was filled on Chi X @21c was simply because Commsec finds trades at the best price available to you across both platforms. However if you were using Westpac, which doesn't have Chi-X, your order would have filled at 22c.

If someone programmed a bot to sell shares for lower than what people are willing to pay, that programmer would be fired quick smart.