Title text:
That was quicker than usual! The cabin's sprinkler system often makes it really hard to keep anything lit.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3114/
It didn't understand the 4th panel as an explanation of the joke but just an emphasis on how absurd it is that the character planned all along that his installation will be wrong.