Note also that the product lets people "choose from multiple sponsorship tiers, with monthly payment amounts and benefits that are set by the sponsored developer." That opens up much bigger pots of money. E.g., a premium support tier is totally justifiable for software that I'm building key code on. That could be $25-100/month, no problem.
And there's a reason conferences sell sponsorships: lots of businesses see it as good marketing to support things that are visible and important to a community. For example, I could totally justify a big-dollar project sponsorship ($1k-5k/year) as a recruiting expense if I want to hire people who already know something important to our work.
If you said the money is going to GitHub, and it's actually going through GitHub, that's lying.
I think it will probably show up differently on credit card statements anyway, so people won't be tempted to do this behind the backs of the accounting department.
GitHub will know two very interesting data points: the relationship graph between projects and dependencies and the sponsorship level of those dependencies. Combine that with Issue and PR activity, and it could therefore identify which highly-depended-upon projects need funds the most and nudge accordingly.
"Did you know that you and 2,500 other codebases depend on $lib.js? Why not pledge $25 to support its development?" That sort of thing.
I, and a lot of other people, cannot get accounting at where I work to cut a check for that kind of sponsorship. I can get accounting to cut checks for things that matter. Typically if someone asks what's the difference between paid and free, and you answer, it supports Active Directory, you're most of the way there if you can show a business need for the product. But just a straight-up donation? It's a hard sell.