Social scientists especially tend to deal with averages. And often forget in public talks to provide the appropriate nuance to their statements. So, I would guess Dr. Fisher is asserting that the average person has a hard time being in - what she calls - romantic love with more than one person at a time. Not only are we talking about persons on average, but situations on average as well. So, even an average person within a highly unusual circumstance might have an experience well outside the norm.
I have no disagreement with the notion that - on average - we can't have romantic love/NRE for more than one person at a time... while recognizing that there is variation in both people and situations which may lead to that "rule of thumb" being violated.
Overall, the TED talk resonated with me. The notion of there being three kinds of love (romantic, attachment and lust) is interesting. Doubly interesting in poly scenarios.
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”