The reason she says that is because 70% have a yellow undertone.
HOwever Yellow concealer is great for dis -colord skin ONLY! YOu should use a concealer in your skin tone category.
About foundation... some makeup artists say to add pink to yellow skin, and add yellow to pink skin (to neutralize) and other makeup artists say to just match perfectly!
I think its good to have everything match my neak, and since I am pink, i use a tint of yellow underton in the center of my face so I look a bit warmer.
the best bet is to just do one on each side of face, and decide wich one you like better.