Why does cooking oatmeal starting with cold milk make it creamy?
Flour and cornmeal are well known to clump when added cold to boiling water. Such clumps arise when starch molecules unball and forming a mesh that traps other starch molecules, preventing them from hydrolysing in the same way. Hence lumpy gravy and sauces.

For oatmeal I've observed similar clumping behaviour, but not to the same extent. Anyway I suspect the same mechanism is at work. Since starch hydrolysis is the main reaction making oatmeal creamy, I'm not surprised that slow and steady heating is considered best.