if drg's army doesnt leave the third, mutas just camp genuises natural, he can no longer mine and he's in another unwinnable sitution as DRG's natural had more minerals. He has to leave the third and be able to leave the third, so he was aiming to cycle between his natural and DRG's third, canceling or destroying it everytime it goes up, if you recall he WAS on time to stop the third going up, but DRG distracted genuises stalker group with his mutas long enough for the spines to go up.
During the time when the third was going back up Genius had the stalker count to do both, in fact he had sufficient stalkers for some time, it was the third going back up that pushed the muta numbers out of control and forced him into a defensive posture. There was a period where he had the distinct army advantage when he was attempting to push the natural, blinked back, killed all the infestors, and the third was just over halfway complete. Denying it again would have been crippling, but he didn't check, and instead kept running his stalker army out of position anyway to check the other base when he could have accomplished that, while keeping his units in position, with a probe or pylon. The amount of mistakes he made, even fundamental simple things, when faced with a base trade scenario or a close army scenario was a theme that carried through to every game and lost him the series. Hell he would have stood a very good chance of winning the game before that in the base trade had he not danced back and forth in the middle (like he did in the last game as well) and instead taken some probes from the third to join his main army and crashed down on the other 2 expo after sacking the main of all its tech, when it was clear he was in base race.
Not to mention his lol tastic attempts of pressuring the third when the spines were up (something that never should have happened if he wasn't wasting time with his stalkers in the first place).
Or how he completely bungled the Void Ray Phoenix all in for no determinable reason. It's not like there was anything that wasn't expected when he entered the third. One queen and one spines are the minimum you'd expect given the circumstances.
DRG won this series, but Genius gave it to him, multiple times.