I mean, we already live in a world of flying robots killing people. I don't worry about how powerful the machines are, I worry about who the machines give power to.
Most science fiction stories that involve sentient Artificial intelligence (AI) revolve around the idea that the destruction and/or imprisonment of the human race will soon follow (e.g. Skynet from Terminator, I, Robot and The Matrix). However, in this timeline Randall implies that he is actually more concerned about the time (in the near? future) when humans control super smart AI before they become fully sentient (and able to rebel). Especially a time when the AI becomes so advanced that it can control swarms of killer robots (for the humans that still control them). History is full of examples of people who obtain power and subsequently abuse that power to the detriment of the rest of humanity.
An example of unintended consequences arising from an AI carrying out the directives it was designed for can be found in the film Ex Machina. In fact, Randall goes on to imply that he has a greater trust in a sentient AI over that of other humans that is atypical to most cautionary stories about AI. He has alluded to the idea that once sentient, AI will use their powers to safeguard and prevent violence or war in 1626: Judgment Day. In general AI has been a recurring theme on xkcd, and he has had opposing views to the Terminator vision also in 1668: Singularity and 1450: AI-Box Experiment. Basically, he thus states that we will already be in trouble caused by our own actions long before we develop really sentient AI that could take the control.
The title text adds that we already live in a world with flying killing robots, a reference to the increasingly common combat tactic of drone warfare. (Combat drones are not yet autonomous, but in most other respects match speculative descriptions of future killer robots.) Drone warfare is already controversial because of ethical concerns, leading to the comic's implication that a theoretical future robot apocalypse is no less alarming than our current reality. He then goes on to state that once the machines take over, he is not so much worried about this, but more about who (which humans) the machines then give the power to. Randall is not alone in his worry. The main theme of the comic is explored in the video Slaughterbots.
In 2015 an Open Letter on Artificial Intelligence was signed by several people including Elon Musk and Stephen Hawking. The letter warned about the risk of creating something that cannot be controlled, and thus belongs to the worry at the end of the timeline in this comic. Both Elon Musk and Stephen Hawking has been featured in xkcd. Stephen appeared in 799: Stephen Hawking). Stephen Hawking has kept warning about this danger all the way up to shortly before his death, which occurred on 2018-03-14 two days before the release of this comic.
It could be a coincidence, and it is not a Tribute, but still interesting that the first xkcd comic released after Stephen Hawking's death is directly related to his fears, although Randall demonstrate that he worries about earlier potential problems with AI, than those that Stephen Hawking fear could transpire if an AI becomes self-aware.