Large-scale observations require a ton of processing and interpreting. So much so, in fact, that there was some debate as to whether the so-called "Great Attractor" even actually existed.
Interacting, independent universes are a hallmark of string theory. String theory has another characteristic for which it is notorious, though, and that's a lack of strong experimentally verifiable hypotheses. String theory is absolutely fantastic at ex post facto explanations of experimental results, but it spectacularly crashes and burns when it comes to predictions of results as-yet unknown.
To illustrate the problem, the so-called "Standard Model" of physics has 14 fundamental constants, with no apparent reason for each of them having the particular value that we measure. There are certain conjectures, but no standard explanation, so you might consider that number 14 to be the amount of information "missing" from the Standard Model. That is, we have to add those 14 ingredients to the equations of the Standard Model in order to describe everything else about the Universe in actual hard numbers instead of placeholders.
I read an article recently that described one point of view on String Theory in which that theory is missing not 14 but 10^(some really big number) of ingredients(!) in order to nail down all of the physics it describes into hard numbers. With that many degrees of freedom, of course you can massage the numbers hard enough to make any result you like pop out, but it doesn't mean you have created anything useful or even true. So, yes, some string theorist may have shown that string theory massaged a certain way can produce interacting universes that look like The Great Attractor, but it's very hard to commit to that flimsy linkage between theory and experiment.
Even just the experimental outlook by itself is not much rosier. Due to the very nature of strings and the excellent agreement with experiment that our existing theories already have, no experiment looking for evidence of the "stringularity" of the Universe is going to be a smoking gun. Rather, any such an experiment will be subtle, require mind-blowing precision, be subject to a lot of confounding factors, and will be susceptible to widely varying interpretations.
To answer your last question, just for an example, another hypothesis that requires muuuuuuch less theoretical "architecture" than string theory and is actually pretty difficult to rule out experimentally: chance. Our observable Universe just happens to contain a really big clump. Tada! Tada?