Actually, i think the History Channel are talking out of their collective ass on this one, as is so often the case with that all-too-often unreliable source. I have read, many, many years ago, that the symbol is Masonic in origin. The Masons were very pervasive in American society at the time, and that included a great many of the "founding fathers."
Below is an image of George Washington laying the cornerstone of the Capitol. Note that he is wearing a Masonic ritual apron, and that the pyramid symbol appears on that apron. This image is not that clear, so if i can find more, i will.