Hmm, I'd have to get them to clarify because that makes zero sense.
The viscosity has nothing to do with a flat tappet camshaft. On top of that, the multi-viscosity only means that the oil behaves differently when cold and hot. So, at hot cruise, a straight 30W oil behaves exactly the same as a 10W-30 oil.
Break-in is a different scenario and typically the Brad Penn break-in oil only comes in a straight 30W. However, they (along with Gibbs and a few others) make a 10W-30 for flat tappets that I use on a regular basis. As a matter of fact, if I had built your engine, I'd be recommending Penn non-synthetic 10W-30 oil, or a Valvoline VR1 non-synthetic 10W-30.