> What’s up with this internet argument style where people think that pointing out an obvious exception is a valid retort?

Because it's not so obvious exception as you think it is, in fact it's the exact opposite, it's a very common type of allergy (Mediterranean, middle east, asia) and usually overlooked by many, that's why.

> Are you seriously suggesting that they needed to provide a disclaimer about allergies to their general statement about lentils being healthy?

Anti-nutrients (beside the allergies) in lentils/legumes is a real issue, so the "lentils being healthy" statement is highly debatable.