Title text:
Another group of mathematicians is working to put an upper bound on the number, although everyone keeps begging them to stop.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3137/
If a number is long enough human won't be able to perceive it all at once, so in fact there are only finite number of cases we need to check.