He's trying to say some people get so much back in credits it's more than they pay in fica, but it's probably a teeny tiny amount of people that actually get that. Like you'd have kids and max the EIC and get child tax credit and make very little as well like minimum wage.
Fica is an extremely regressive tax in my opinion but it stands that way because it's technically insurance premiums for a fixed benefit amount, that's why everyone pays it and it phases out. People argue it's not regressive because those who make less income supposedly receive more in benefits on the other end. At the end of the day though social security is hardly merit based, it's really like basic income for people over a certain age. Thus is should just be like regular taxes, maybe everyone should pay it starting from $1 but I don't believe it should be capped on earnings on the other end. You could fix a ton of social security solvency issues by just raising that threshold but congress will never go for it as it would be a gigantic tax increase on rich people.
Regarding the senate tax plan it seems like a bum deal for the middle class. I expect my taxes to go down a few hundred bucks, hardly anything but it's better than nothing I suppose. If it were up to me, well my full tax plan would be a much bigger overhaul including raising the cap on income taxed by fica, not taxing long or short term capital gains at all on the first 10k or so to encourage middle class people to invest outside of their 401ks, increasing child tax credits, but mostly I'd just leave current deductions and brackets alone and just change the percentages. Make the 10% bracket 7% and the 15% bracket 13%. Everyone who makes over ~47000 single (the top of the 15% bracket for single filers taking standard deduction plus one exemption) or ~90000 married would get the exact same tax break, around 900 for a single filer, around 1800 for a joint filer. People at the bottom would still get relief though it'd be much less of course cus it's still percentages but this would greatly benefit the middle class as those in the middle of that bracket would see the biggest percentage gains.
If your goal is to give the lower and middle classes a tax break without just giving away a $500 credit to every person like we got from bush that one year, it seems like an obvious solution to me to simply reduce the percentages in the lower brackets, but clearly this doesn't benefit rich people enough so it will never happen.