There is a distinction between solving ARC, creating AGI, and creating an AI that would represent an existential risk. ARC is a stepping stone towards AGI, so the first model that solves ARC should have taught us something fundamental about how to create truly general intelligence that can adapt to never-seen-before problem, but it will likely not itself be AGI (due to be specialized in the ARC format, for instance). Its architecture could likely be adapted into a genuine AGI, after a few iterations -- a system capable of solving novel scientific problems in any domain.
Even this would not clearly lead to "intelligence explosion". The points in my old article on intelligence explosion are still valid -- while AGI will lead to some level of recursive self-improvement (as do many other systems!) the available evidence just does not point to this loop triggering an exponential explosion (due to diminishing returns and the fact that "how intelligent one can be" has inherent limitations brought about by things outside of the AI agent itself). And intelligence on its own, without executive autonomy or embodiment, is just a tool in human hands, not a standalone threat. It can certainly present risks, like any other powerful technology, but it isn't a "new species" out to get us.