The article specifically suggests writing the key down in a notebook. A single incorrect digit and the scheme fails much later in a way catastrophic to the user.

▲

whyever 6 days ago | parent [-]

Signal asks you to repeat the key immediately before even enabling backups. It cannot fail much later unless you modify the digit after the check.

	▲	upofadown 6 days ago \| parent [-]
		A longer key makes typing a bunch of characters back into the phone much less usable.