Hi,
I've worked extensively on Google Analytics Premium and tag management platforms, namely Tealium and Google Tag Manager for the past 3 years, therefore I could assure you that I've the relevant expertise in order to get the Mailchimp user ID mapped into the custom dimension in your Google Analytics account. However I also have a suggestion regarding this, rather than following the approach given in the article I strongly suggest that you consider implementing User ID in Google Analytics which would help you get a much broader view of each individual users journey across your site. This would require a bit more work but provides much greater value and more accurate data.
Let me know if you'e further clarifications
Cheers
Yoosuf