From the Twitter link:

"I was a TVC — which stands for “temps, vendors, and contractors,” a designation within Google for workers who aren’t full-time employees and are hired by third parties companies, not by Google itself. We do the same work as full-time Google employees — I worked right alongside them — but we don’t have the same corporate benefits."

This is another way to screw workers, allowing large companies to hire "employees" without having to pay benefits or FICA taxes. If Google managers are directing and controlling these contractors as if they were actual Google employees, doesn't this violate IRS rules on independent contractors?

For example, if I hire a company to do my video productions, that's fine. But if that company's employees all are required to work in my building, and I supply the desk, chair, computer, and have a employee/manager who directs their daily actions, they are not functioning as independent contractors according to IRS guidelines. In that case, the contractors might be able to sue Google for benefits and past taxes they've had to pay that Google employees don't have to pay.

TVCs are W-2 employees of a company, with the relevant taxes paid and benefits present.

Of "a" company but not Google. It's unclear how the benefits of Google and the other company compare. I'd guess not well.

Probably not, but there's no legal issue with a W2 employer directing you to work on the campus of another company whose own employees have better benefits.

Didn’t this already go through the court system long ago because of Microsoft and this is why these kinds of workers can only work for a particular company for 11 months and then have to be off for 3 months before they can get a new contract with the same company?

The two-tier system is bad, but it's not at all Google specific. It's extremely common.