Not exactly wrong, but incomplete and not really up-to-date on how preclear fits into Unraid these days.
Also note that the only time Unraid requires a clear disk is when adding it to a new data slot in an array that already has valid parity, so parity will remain valid. A replacement disk, for example, doesn't need to be clear since it will be completely overwritten by the rebuild anyway.
And, Unraid will clear a disk in that one scenario where it requires a clear disk. The historical reason for preclear is because Unraid would take the array offline when it cleared a disk, so preclear was invented to clear the disk before adding it so Unraid wouldn't need to take the array offline. Additional testing functionality was builtin to preclear to help weed out the "infant mortality".
Recent versions of Unraid do not take the array offline when it clears a disk, so the only function preclear has these days is testing.