The primary meaning of 板 is 'board'. So why does 老板 or 'old board' mean 'boss, chief'?
Good question. In Traditional Chinese, boss is called "老闆" instead of "老板". "闆" is simplified into "板" but they have different meaning, that is, they are two totally different words. Therefore, "闆" doesn't mean "board". "闆", reads pàn here, means spy through the door. See http://www.zdic.net/z/27/js/95C6.htm.
According to MOE Dictionary of Taiwan, the word "老板" is also used nowadays. See http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&searchid=Z00000060086.
As for this question, it seems that there are different opinions and you shouldn't trust them blindly.