Mustard seed parable as a challenge to fundamentalist faith.

I was reading a book yesterday and it said that the parable of the mustard seed was a challenge to this one notable fundamentalist prof's faith:

He put another parable before them, saying, "The kingdom of heaven is like a grain of mustard seed that a man took and sowed in his field. It is the smallest of all seeds, but when it has grown it is larger than all the garden plants and becomes a tree, so that the birds of the air come and make nests in its branches." (Mt 13:31-32)

Botanically, the mustard seed really isn't the smallest seed on earth, so the guy was troubled that Jesus didn't have perfectly accurate botanical knowledge. He finally was comforted, though, because he realized that Jesus was talking about all the seeds you'd sow in a garden, and not actually about all the seeds on earth.