That's a silly claim.
> Picking colors from a photograph is a quick way to choose muddy colors.
That's a little less silly. It's going to depend on the photograph.
For colors, I'm assuming that while you can make exactly the color in the photograph, the colors will mix on the canvas leading to a muddying of the colors. I'm not a painter, but I'm guessing you'd want to create brighter colors so that the final result matches your intended color. (And also it's easier to dull colors than to brighten them.)
This is not about how a painting looks online, this is about how the painting itself comes out.
Unfortunately, even well-known college art programs are not always rigorous in teaching theory. Art education is also a far different subject than being an artist. Bad habits are easy to create, and this app encourages shortcuts that create bad habits.
The same goes for objects, and photos typically capture only a limited subset of this.
"to learn how to create contrast and depth in your paintings"
The creator talks about learning core art skills that a working artist should already have.
- wrong colors, because of the lower dynamic range of a camera compared to the human eye
- lens distortions