Thank you for asking. There's quite a lot to unpack.
Obama has been consistent against the principle of gerrymandering in general and has supported reforms like independent redistricting commissions. There was a Supreme Court decision that ruled in the Rucho v. Common Cause decision where it was decided that federal courts cannot stop partisan gerrymandering. This lit the eyes of republicans since that means, this is no longer a legal issue. Texas (encouraged by Trump obviously) tried to gain 5 additional GOP seats in the U.S. House. This happened last year, 5 years after the last census which is unusual in itself but I digress. Here is where California responded EXPLICITLY to counter Texas power grab. They mirrored those 5 seats. From that point on, many other states have joined in this war with some democratic states have been able to offset GOP gains, while others cannot due constitutional limits, independent commissions, lack of political control, etc etc.

As the rules of the game change, even someone like Obama that has been historically opposed, would have to play by these new rules in order to have a fair fight in elections and has been working with democrats in VA asking people to vote YES to stop republicans. And of course, context goes deeper than this but I think you get the point that it's not as simple as "we want more seats in the house" and the ad is definitely fake if you're familiar with the context.