Why would they pay a near-nobody to advertise? That's not how this works. The money goes to Medium to host content as a native advertisement, or the money is spent on ads and promotion elsewhere to get more clicks on this article. Neither of which has to be disclosed.
Also - what a surprise. The article writer owns a marketing and advertising consulting company! With a specialty in social referral campaigns and native advertising! Which is coincidentally exactly what GiveDirectly is focusing on right now.
The lack of money directly exchanging hands between the person who wrote it and the beneficiary doesn't mean it isn't advertising.
It's interesting how this disclaimer says that Andrew isn't being compensated by GiveDirectly and its affiliates etc. It doesn't say anything about whether any of the affiliates of Andrew (such as his business) received compensation from GiveDirectly, which is the much more likely scenario. Regardless, it's easy enough for anyone to claim they're writing something as an individual and not as a service to their own company.