This is the most laughable assertion. As discussed earlier this month, if both sides will win the seats they have likely or better, the divide will be Republicans 211 Democrats 170. The Democrats will have to win 89% of the remaining races.
Those races aren't all 50%-50%. More favor the Republicans, as many are Republican incumbent seat. But even if we assume these races are 50/50 the odds of the Democrats winning 89% of the competitive seats is over 5,000 to 1. That's, in essence, 0% not 50%.
Circumstances could change to favor the Democrats in the House, although I don't see how since 2nd term Presidents have very short coat tails. Pelosi, of course, isn't talking about circumstances changing to favor the Democrats. She's talking about today.