The article doesn't tell you but going to the actual page[1] tells you it's restricted to US only.

> The Google Store Promotional Codes Terms apply. Offer2 only available in the United States while supplies last.

[1] https://pixel.google.com/developer-discount/

On the actual page (not the OP page link):

"The Google Store Promotional Codes Terms apply. Offer2 only available in the United States while supplies last. Privacy Policy"

You'll certainly see 'Not available in your region' if you try to checkout with the code.