Interesting supposition. The multiverse is just a hypothesis, there's no proof the concept is real, so this idea is more in the realm of metaphysics than real science. Still, humanity doesn't understand the quantum world yet, and it is building tech that utilizes it.

On the opposite end of the scale is dark energy & dark matter, which shows we don't really understand the universe at the macro scale either, yet we've been existing in it for millenia. Whatever is real, is just as real as it ever was, whether we understand it or not.

So perhaps this extra computational power is coming from "somewhere" we don't understand. If you thought AGI was scary, AGI powered by computing coming from a mysterious unknown "somewhere" sounds even more troubling.