This isn't marketing this is just how the media decided to spin this story. Saying "Apple was caught off guard" makes the story more engaging than just saying Mac minis are sold out for 4 - 12 weeks after launch.

We don't know if Apple misjudged the demand or couldn't source enough parts to match the initial demand. Another explanation could be that they decided it wasn't worth it to increase the production capacity to match the higher demand at launch because that capacity would go unused later down the line.