You're right that we aren't certain that changes in tax rates alone might increase inequality, but the author is also speaking about that the cuts to welfare programs and social services that will probably follow the tax bill. The sentence that you elided from your quotation is:

"The dramatic cuts in welfare, foreshadowed by the President and Speaker Ryan, and already beginning to be implemented by the administration, will essentially shred crucial dimensions of a safety net that is already full of holes."

This is more than mere speculation--this is what the Speaker is saying he wants to do[1]. This is an important part of the background for this analysis.

[1]https://www.washingtonpost.com/news/wonk/wp/2017/12/01/gop-e...