Cost of item = 10
First discounted cost of item = 9
=> First saving = 1
Second discounted cost of item = 6
=> Second saving = 4
Second saving is 4x first saving.
(Edit - formatting)
But that's 4x the savings compared to another saving. I suppose you've upped the pedantry and are technically correct, but that's a pretty narrow use case and not the one used in the article.