Follow the link
https://xkcd.com/2030/The alt-text provides more comment, just hover on the image to see it.
They are not being sarcastic about people worried about inaccuracies arising from computerized voting, they are saying software is always buggy, and should not be trusted with something critical.
It's from 2018.
https://www.explainxkcd.com/wiki/index.php/2030:_Voting_SoftwareThe comment about blockchain is that it is a new technology, and at that point in time it was the latest tech buzz word that everybody was trying to find a way to use somehow. It's a comment that anybody saying they've solved a problem using blockchain is not someone who can be trusted, it was just the latest industry buzzword. Much like "web 2.0" in the early 2000s, and "cloud" for the last decade or so.