The point being made, though, is that while of course how a person chooses to live is a lifestyle, which includes many things, but "a lifestyle" is not necessarily the same as "the lifestyle" as if there is one agreed-upon, mutually predetermined way to be in order to qualify as having that kind of lifestyle.
Someone can say, "I am polyamorous" or "I lead a polyamorous lifestyle," where it still remains open to interpretation and is obviously a choice that person made to incorporate polyamory into their own individual lifestyle -- but it simply assumes too much if that person said, "I live the polyamorous lifestyle." Then, someone would hear that and say, "Oh, what is the polyamorous lifestyle?" or, "Oh, I want the polyamorous lifestyle." And what would they do, model their lives after the person who said it in the first place? Then they wouldn't be creating their own way of living and being polyamorous, and if they run into someone else living polyamorously but in a vastly different way, they could think either that person or themselves are doing it "wrong." So, to say polyamory is a lifestyle to adopt, when it is simply the choice to love more than one, can contribute to erecting barriers and misunderstandings.