Rust Brain Teasers: "Stacking Boxes" heap vs. stack performance explanation

Title: Rust Brain Teasers, page 47

“Because of the extra steps required for heap read/write access—particularly with frequent allocations—accessing data on the heap can be a lot slower than accessing data on the stack. Why? Because the CPU’s memory cache will try its best to keep your heap data available.”

This explanation doesn’t really make sense - both heap and stack are cached, and in both cases “the memory cache will try its best”. I think this might be trying to say something about stack memory has better locality, so the cache has an easier job, but even so, the above paragraph struck me as misleading and/or confusing.