"keep that clock speed rock solid and therefore be able to synchronize events across those 1.57 million cores"
You're saying the cpu cores are all clock-synced, over a warehouse full of racks, at GHz frequencies? Uh huh.
Dealing with clock-skew across a *single* *core* is non trivial. Who fancies a whip-round for a copy of Hennessy & Patterson for the author? Call it an investment.
And the 18th core is there to 'increase yields' and is typically disabled, make naff all sense. If you build it, you enable it. If you can't get all working, the chip goes cheaper. That makes sense. So perhaps there's something else going on like it's disabled to keep within it's power/thermal limits. Or there's some sly marketing connivance happening.