Well, let’s not forget that before Toby left he told Kate “only boob juice” (ugh) until he got home so he wouldn’t miss the big first bite. He communicated to her what a big deal it was for him (perhaps he was overplaying it, but she wouldn't know that) and she was reacting to that.
Then unexpectedly she let the moment get ruined (her internal monologue no doubt) and felt guilty. Then Toby got baby Jack to eat, seemed so happy, and didn’t want to ruin it again.
But then he’s so excited about “Jack's first bite” video getting likes, so she felt compelled to be honest. Maybe so he won’t feel stupid later, having made a big fuss, finding out the truth. It’s a small thing all around, and it’s not a big deal if she had kept quiet or decided to tell. Either way it’s not a big deal, and just a new parent thing to get through.
She also fully recognizes her frustration with his weight loss is rooted in complex feelings and history, much of which she owns and is her self-described “evil” feeling. She was very emotionally healthy about her reflection on it. And she chose not to confront him about the text in the middle of the holiday and on the heels of her “avocado-gate” misstep. Also thoughtful and self aware.
I don’t think she deserves a lot of the criticism she gets.