I disagree. If “what you know and love” is your product (and it should be), than THAT is what you need to be focusing on. The most successful entrepreneurs are the ones who realize there are far too many aspects to building a great product for one person to handle alone, and surround themselves with smart, driven people who are passionate about other specific areas of focus that will ultimately drive the product forward.
Apple needed Steve AND Woz. Google needed Larry AND Sergey. Facebook need Mark AND all the people he stole ideas from ;)
While I agree that sayings may or may not always reflect reality, I think in this case, the one I mentioned certainly does.