By "a heavily optimizing JIT" I mean "the specific mode of a larger JIT-enabled engine that compiles slowly but emits good code". So IonMonkey, Crankshaft, SpeculativeJIT/FTL are all JITs that live inside a larger engine that switches between them as needed. In JS land we usually call the individual modes "JITs" even though they may (or may not) share a bunch of code.