Hiya, author here! I wrote the article to mostly encourage others to sign up to these charities to help accomplish their mission, the data generated as a by-product is just a cool side-effect!
Because one can does not necessarily mean one should.
At present it's clearly not scalable to be sending GDPR requests one person at a time. I think perhaps if this kind of incentive brings more people to sign up than traditional donor recruitment drives (which is often much more than handling a request for data!) then these charities will provide easier, more scalable ways to access this data :)