As the poster doesn't this gives Trump the power to manipulate markets with nothing more than a tweet?

You could argue he already has this power, but before the message needed to resonate with a large collective of people. Any public figure (like Warren Buffet, etc) can express an opinion that might effect the stock they are commenting on.

However as this group of people automate trades on only the contents of his tweets (and not say the contents of the tweet as well as past company performance, current POTUS approval ratings, dividend plans etc) he gains a scary amount of control over the markets.

To put another way. The only people who know what he will tweet earlier than these bots are Trump and maybe a select group of people he tells before hand.