According to Everett's seminal paper "The Theory of the Universal Wavefunction", the Many Worlds Interpretation is motivated by considering the problem of multiple observers in the universe. From the Introduction of this paper:
The question of the consistency of the scheme arises if one contemplates regarding the observer and his object-system as a single (composite) physical system. Indeed, the situation becomes quite paradoxical if we allow for the existence of more than one observer. Let us consider the case of one observer A, who is performing measurements upon a system S, the totality (A + S) in turn forming the object-system for another observer, B.
Here in defense of his considerations, Everett starts with the notion that treating a single observer as special leads to some problems when you add in another observer; in this case, the original observer A is observing system S, and some other observer B observes A and S together.
This core of the introduction is based on the idea that there are two processes (using Everett's labeling): Process 1, an indeterministic process, is probabilistic and occurs when an observer makes an observation; and Process 2, a deterministic process, occurs when observers are not looking. Process 1 is a reference to the Born Rule, the probabilistic outcome that occurs due to wavefunction collapse. Process 2 is a reference to the Schrodinger Equation. Loosely, Everett considers that when B observes A+S (observes A and his observations of S), then B is compelled to use Process 2 to describe the situation. But when A is just observing S alone, A is compelled to use Process 1. This leads to the paradox Everett introduces. The MWI stems from resolving this paradox by rejecting Process 1 (meaning, rejecting that it's a "real" thing).
For example, if Everett is observing Schrodinger opening his box, then to Everett, there appears to be a superposition between Schrodinger seeing a living cat and Schrodinger seeing a dead cat. In the spirit of the paper, Everett then concludes that this is the actual reality... there's a universal wavefunction here that is in a superposition between Schrodinger seeing a living cat and Schrodinger seeing a dead cat. The Schrodinger that sees a living cat simply doesn't interact with the Schrodinger that sees the dead cat, but they both exist in a universal wavefunction.
I discuss this here so you can see how the assumption that particular observers aren't special leads to the MWI through Everett's eyes (and you can walk through the introduction to Everett's paper and compare to this description if you like). Compare that to this from the blog:
It’s as if our observations roll the quantum dice and influence which course through the multiverse each individual consciousness takes
Sackler here is treating not just the particular branch he's on as special, but the path "through the multiverse" leading to that branch as special. Suffice it to say this adds to the MWI. But more to the point, given this view of MWI's core justification (at least as given in Everett's introduction), it seems to conflict with the "spirit" of MWI in the sense that MWI is questioning the notion that particular observers are special, yet Sackler is presuming that very thing.
