Some dude on their forum posted a workaround to redeem it, and I'll just list the simplified steps to do so, just make sure that your Steam account is already linked.

Visit this page https://community.amplitude-studios.com/profile/me/rewards/badges/180

then, you should see what's shown in the screenshot I provided

search and click on Connect your account to Steam in the left side menu

Click on Redeem in the new loaded content

Nope, just says "An error occured with your last action. Please reload or contact an administrator if this error persists.".

Thanks mate, it worked. 

I guess since it's an old game and not many people claim it, the normal way broke and the devs haven't realized it.