"Two of the factors were warming sea surface temperatures in the Atlantic Ocean and high radiative forcing, which occurs when the atmosphere traps more energy from the sun than it radiates back into space..."
IOW, the solar constant is variable, the sun was just a hair hotter 800 to 1600 A.D.
That warmed the Atlantic, and pumped a wee bit more energy into the atmosphere than could passively radiate out into space.
That sounds like the era that was called “The Medieval Warming Period,” when the Earth was four degrees warmer than now.