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"

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/

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