There was a bulge in the south of Army group center but Guderian and von Bock argued that Army group south was strong enough to defeat it on its own. Guderian pressed for an advance on Moscow as soon as possible.

Army Group South was not in a position to destroy the Kiev salient on its own. Without assistance from Army Group Center, they would be unable to mount a double envelopment against the Red Army. They would have to lever them out of Kiev after a bloody, time-consuming, and expensive battle. The Southwestern Front would be able to retreat in good order and avoid being encircled and destroyed. They would be able to tie down Army Group South and prevent them from advancing further. The battle of Kiev would end with relatively few Soviets being taken prisoner, compared to the incredible total of 650,000 men.

Supplies needed to be hauled, sure, but the same was true for the diversion to Kiev and Leningrad.

As far as the two panzer armies of Army group center were concerned, month of August was wasted maneuvering behind the Soviet forces in Kiev and September was wasted in destroying the Soviet forces in Kiev.

Army Group Center was conducting an operational pause in August. They brought a halt to all major operations because they had outrun their supply lines. This was part of the plan for Operation Barbarossa. The Germans depended on the railways to bring supplys from the Reich to the frontlines in Russia. They were not getting enough trainloads of supplys to carry out any major operations in August. The logistical situation had barely improved by September. Army Group Center literally did not have enough fuel or ammunition stockpiled to go after Moscow. They needed to spent a whole month building up a stockpile to do this. In the meantime, they sent a portion of their forces to go after the Kiev salient. This was all they could do.

Say what you will about Hitler, but he made the right decision in this instance. Halder, Von Bock, and Guderian were completely ignoring the logistical aspect of the plan they were championing. They wanted to go pell mell after Moscow, but if they had done so, Army Group Center would literally have came to a stuttering halt after a hundred miles or so. Logistics are boring, but they place a hard limit on what operations an army can execute.

By the time the offensive against Moscow resumed in October, the high time had passed. The Rasputitsa set in and the whole army got bogged down in mud. It still managed to score stunning victories at the Mohzaisk-Vyazma line but it got paralyzed by winter afterward.

If the Germans had launched Operation Typhoon in September instead of in August, they would have been brought to a halt by supply shortages instead of by the mud. Armys cannot run on hope alone. Tanks and trucks need gas, artillery need shells. Creating the Vyazma-Bryansk pocket 1 month earlier is not going to change the outcome of the war, not if they peter out before they capture Moscow. In fact, this is a worse tradeoff than what historically happened, because the Kiev salient hasn't been destroyed yet. Southwestern Front will still be in action, and causing all kinds of problems.

Hitler made an error in judgment. He underestimated the value of Moscow as the railway hub of the USSR. He assumed that the loss of the rich Ukrainian industrial zones would've crippled Soviet war-making capabilities. What he and German intelligence did not realize was that the Soviets would be able to resume and indeed expand their industrial production east of the Ural mountains.

The purpose of Operation Barbarossa was NOT to capture Moscow: It was to destroy the Red Army in encirclements. Hitler spelled it all out in Fuhrer Directive 21, back in December 1940. Halder, Von Bock, and Guderian were all missing the point. It was not enough to simply take Moscow off the march. They needed to annihilate the Red Army before they took Moscow. Step 1, and step 2. It has to be done sequentially, otherwise the Wehrmacht would have ended up in the same situation as the Grand Armee. They would have held the capital but faced an unbeaten enemy ready to fight on in the winter.

Hitler made the right decision. The battle of Kiev was a terrible blow to the Red Army. The loss of Southwestern Front forced the Soviets to send their reserves to the Ukraine in a vain attempt to plug the huge gap that had been torn in their lines. The loss of industry in the Ukraine was not crippling, but the loss of farmland absolutely was. It placed the Soviets in a very difficult position, as they were forced to cultivate new farmlands in the East. This consumed lots of manpower, and the farmland was not as productive, which put them at risk of experiencing a famine. (This risk became a reality when the Germans launched Case Blue in 1942)