I think the implication is not that jQuery is bad for DOM manipulation, but that you should leave DOM manipulation to React to have a 1:1 relation between the held state in React and the actual DOM. In other words: Do not manipulate the DOM outside of React.

This is a React style guide; which implies that React does all (if not, most) of the DOM manipulations for you. jQuery, as a library, have useful utilities that that do not do DOM manipulations.

React keeps all of the "DOM" stuff in memory. Thus it is inefficient and unnecessary to use jQuery to do anything to the DOM.