As noted it was mostly a myth. What is true is that the Soviets moved most of their heavy armor to the West, facing the Heer. The thing is, that the didn't NEED their heavy armor, especially fighting defensively, against the IJA.
The IJA was a light infantry force, quite weak in armor. In 1941 the entire IJA had three divisions worth of tanks. These were also very poor example of the art. The best that was in heavy use was the Type 95 (37mm main gun, 2x 7.7mm mg) that was utterly helpless against the Soviet BT-5 & 7 light tank, something demonstrated in the border clashes at Khalkhin Gol where the Soviet tank's 47mm gun simply shredded the Japanese armor. Half of their total armor was sent South in December of 1941, a full tank group of four regiments (what was called a division in other armies) was with Yamashita, and two regiments were assigned to 14th Army and Homma on Luzon. They had six active armored regiments for the rest of the entire Japanese army, spread across half of China and into Manchuria.
They were also lighter in artillery than the average Western force, with most divisional formations not having anything over a 75mm (their HEAVY A type divisions included 12 x 105mm). A Soviet division of the same era had more guns (100 vs. 84 for the Type A and 66 for the standard B division) and heavier guns as well (12x 152mm, 28x 122mm, with the balance being 75mm) and was about 25% heavier in machineguns, and notably better heavy mg than the IJA (plus the addition of the submachinegun as the war progressed). The Japanese had more mortars, mainly in the 90mm and below size (the "knee mortar" was more of a grenade launcher, but extremely effective in that role).
Probably most critically is that the Red Army understood that victory via superior elan had died in Flanders and Tannenberg. The IJA never learned this. The IJA remained convinced, all the way through to the Surrender, that bravery and selfless courage was superior to shrapnel. They were wrong. That is why the IJA never defeated a "Western" force that was not "colonial" in nature (the closest it got was in Malaya, but even there most of the troops were not top quality, and the leadership was simply dreadful). Even in the Philippines it had a hard time finishing off a mostly militia force (only about 12,000 U.S. troops were "regulars" both Americans and Philippine Scouts) that was utterly cut off from resupply. Once fighting stopped being against cut-off force with poor training and/or fighting in exposed posts, the IJA never won another battle. The British Army kicked their butts, as did the Australians, the Americans, the Indians, and the Soviets. "Western" forces used firepower and maneuver, the Japanese used near insane amount of courage and deception. Exceptionally brave, entirely outmatched.