Not true. Sessions takes himself to the side, selects Rosenstein, then the two of them recommend Comey be fired. Rosenstein even documents all the reasons in a letter. Then when the president does exactly what they recommend, Comey and Rosenstein arrange for Mueller's investigation. In the meantime, Sessions remains a roadblock to cleaning out the whole mess. Sessions could not have been more effective in tying the president's hands regarding the Justice Department that supposedly reports to Trump in any other way except how it all played out.
If you sincerely believe that he would have been more effective if he didn't recuse himself, explain how.
If his goal was to undermine Trump, then he would have maintained more control over the process if he had stayed involved. Because he recused himself, we now have a situation where Mueller is making himself look like a complete fool every time one of the cases he prosecutes has a court hearing.