On tablet, when the highlighter is enabled, one should be able to drag their finger across text on the screen to highlight it. If you had a stylus or Apple pencil, you could then use that to highlight text as you read it.
The current state: when the highlighter is enabled, nothing happens. You have to click on a word. Select the pop-up menu "Select". Then drag the boundary borders to the section of text you want. Then click the highlight icon in the menu. This makes it very very difficult to read a text document and highlight it