The tax cuts ignore this impact on Social Security's finances

[Igel]

2. I thought so, too.

But Social Security benefits are referred to as subject to federal income tax laws, and it seems that when Social Security benefits are taxed that income tax money goes to either the Social Security Administration programs or Medicare.

https://www.ssa.gov/policy/docs/issuepapers/ip2015-02.html

The link in the OP crashes my browser every time, so there's the official .gov reference. But it means I can't read the article to follow their reasoning.