She baked a pie from a recipe, and uploaded the recipe picture of the pie to illustrate what she'd baked. For a member of the White House press corps - as opposed to a member of the public who might be feeling a bit snarky - to deem that worthy of making a public statement is a joke.
Do you know a single individual who makes something, and then shares a photo of what they used as the basis of their creation rather than the product they made?
That's the joke. And it's going over your head.