That slogan is an oversimplification but if I eat a bowl of steel cut at 7am I won't snack or be hungry until 2pm or later. Whether or not I eat this will have a strong impact on my day. Sounds important IMO
If I do eat early my body takes hours to get to full energy again. It literally makes me tired.
My girlfriend doesn't work that way. And I have no idea why this works so well for me.
It's certainly not the most important meal of the day in general. Especially as a societal meme that came directly from an advertising campaign for a cereal manufacturer.