This sounds like a map/territory confusion. "Intelligence" is a concept in the map, used to summarize the common correlations in success across domains. There is no assumption that fully general cross-domain optimizers exist; it's an empirical observation that most of the variance in performance across cognitive tasks happens along a single dimension. Contrast this with personality, where most of the variance is along five dimensions. We could talk about how each person reacts in each possible situation or "island", but most of this information can be compressed into five numbers.

We could always drill down and talk about more factors, ie fluid vs crystallized intelligence or math vs verbal. More factors gives us more predictive power, though additional factors are increasingly less useful when chosen well.

Though a single-factor model works well for humans, this isn't necessarily the case for more general minds. I suspect the broad concept of intelligence carves reality at its joints fairly well, but assuming so would be a mistake.