They use a whey protein - I don't believe the formula contains soy.

Whey allergies are a thing, so a product cannot both "use a whey protein" and be "free of all known allergens".

Doesn't whey protein contain lactose?

There are two types of Whey protein, concentrate and isolate.

Whey protein isolate is a more "pure" form or whey protein that does not contain lactose. Whey concentrate is cheaper, but does contain some lactose.

The site mentions that the formula is lactose free, so I imagine they are using Whey Isolate.

Gotcha. Thank you.

Lactose intolerance is not an allergy to lactose. Most people are actually lactose-intolerant -- in most of history people around the world simply had no need to drink milk after the toddler stage, and those who are truly lactose-tolerant tend to be of Nordic/Scandinavian ancestry, IIRC. Those who are labelled lactose-intolerant (though that itself is a spectrum) can handle a pint of milk a day.

I understand that, but the claim is "lactose free," not "lactose intolerant people can drink this."

For example, vegans would care about this.

Then it must be the green flavor.

It means "we haven't really looked into that too hard."