That doesn't really make sense to me, but that person is very knowledgeable so I wanted to check. Clear plastic does indeed speed the warming of soil, if the garden is in a sunny location, because it allows the sunlight to pass through to the soil, warming the soil directly. It then holds the warmth next to the soil.
Black plastic absorbs the light directly and is itself heated.
It then must warm the air under it and the air warms t he soil. Because weeds can grow very well under the clear plastic in the early spring. The black plastic, by excluding light from the soil, inhibits weed growth. As the season advances and the weather warms, it is important to remember that the plastic is still functioning. Clear plastic continues to warm the soil into the summer, perhaps making it too hot for the plants you are growing.