Tegmark is claiming that the scientist who evolved inside of the otherwise isolated Andromeda system qualifies as an observer; and since there is an observer, the system as a whole is being observed, and hence the state function will collapse randomly -- which will interrupt it's unitary evolution.
Tegmark raised his hand for "those who believe in the Many Worlds" interpretation; they specifically deny the "random collapse" of the state function; instead this is supposed to generate one of the Many Worlds; the result is that each of the superposed states evolves unitarily at all times; apparently there will be Tegmark's world, and the rest of us who remain in the superposed state along with my entangled photons in the lab.
Your question is: "Why does he say this?"; he is setting up a straw man argument, which he systematically demolishes during the next portion of his talk.
There are many problems with his line of argumentation, beginning with the assumption that there can be a state function for the cosmos, or for any really large system. Certainly no one has ever conducted an experiment which shows such behavior, and standard methods imply that the statistically mixed state is the best that one can do.
As one moves further back in time, as cosmologists are wont to do, he must approach the Big Bang; and then of course there is a lot of unknown physics. Perhaps one of the interpretations provides an answer there.
The fundamental problem is that the discussion is about the meaning of a measurement; the use of the word observation implies (to some) a sentient observer. The propensity in the early days of quantum mechanics to appeal to philosophical studies to further certain arguments is the true background to this entire question of "interpretations". As a student, you want to focus on how to do the calculations -- and everybody agrees on how to do the calculations.