Essentially, because the GOP wave of 2010 helped them win a number of statehouses and allowed them to draw the redistricting lines for this decade, they'll enjoy a lasting advantage in House control. According to the memo, that advantage is equivalent to a 7-point edge.
As McInturff points out in the memo, titled "There's no crying in redistricting," that's a reversal of historic trends: In the 1970s and 1980s, Democrats controlled more statehouses, got to draw redistricting maps, and engineered lasting majorities even when the GOP was winning national elections.
"If you began your career as a Republican trying to win the House in the 1970s and 1980s, you would adopt, as I do, the borrowed adage 'there’s no crying in redistricting,' " he writes. "I do not recall a series of commentators weeping then about the huge structural advantage the Democrats had drawn for themselves, and having missed that opportunity, now is not the time for lamentation."