vilas,
Boeing says no such thing. All that their graphs prove is that you will never have a Vmca problem, you'll stall first. The graphs say that you'll stall around 205Kt. So, one less thing to worry about with a V1 cut. Remember that Vmcg tends to drive some scheduled speeds and it usually is above Vmca.
There's no "if" about it old mate, it comes from Boeing certified data. I imagine that there are several thousand copies around. I, too, have wondered how Vmca could be demonstrated for certification!
Just BTW, the notorious PA44 Seminole also has Vmca just a bit below Vs1g in most conditions, hence the AFM warning about intentional single engine speed in big print.