The Redditor who posted this conversation to the /r/gameofthrones subreddit explained it sprung to mind immediately after realizing what was left of Daenerys' army. Interestingly, when Robert said, "If the Targaryen girl convinces her horselord husband to invade and the Dothraki horde crosses the Narrow Sea... we won't be able to stop them," he was speaking of the very same same power Dany was attempting to demonstrate to Jon in this week's episode. Now that Daenerys' army has been attacked by Euron's fleet and stranded at Casterly Rock, she may be forced to take on Cersei with help from the Dothraki.