I would imagine browsers optimize the iterator protocol away for arrays. At least on https://jsben.ch/iJFZ4 for..of is a smidge faster than an old-school loop.
You accidentally included the array initialization in the benchmarked code ("setup block" vs "boilerplate block"), dwarfing everything else. If we're talking about an already allocated array the difference is pretty big:
… but in a way you're correct that this is rarely the actual bottleneck in the surrounding code. Still, "death by a thousand papercuts" and all that. Plus having old-school for-loops as a habit makes it easier to spot the true botllenecks.
... derp, yes. It was way too early in the morning, and I misread "is part of the benchmark" as "is not part of the benchmark"...
In your defense, the "use it for data initializing" suggestion isn't exactly helpful text either.
Also, for the record: I presume you tested on Chrome, but on Firefox the difference between the two loop styles is quite a bit larger (and generally slower than Chrome, so perhaps also a consideration when optimizing for "slowest browser" as the bottleneck).