You have a reference for that?
In 1941 the Soviets had by far the largest number of tanks of any country, most of which were produced in country.
It seems really odd to me that in early 1942 huge numbers of British and American tanks would be in Russia. The Brits had their hands full elsewhere, and the US had just gotten into the fight, was desperately expanding and needed all the armor it could get for its own forces.
They had the biggest number by 1941 but most were absolete mid-1930s types, there were little to none fuel and people to man it. A majority of these tanks were abandoned and captured or destroyed.
Soviet military was in state of stagnation and decay 1937-1941 due to Stalin’s purges. I think some 95% of highest command were purged and more that half total officers. Soviets probably had the best technology and numbers in mid-1930s (for example their SB bomber were faster and flew higher than most German fighters intended to intercept it early in Spanish civil war) but in late 1930s armed forces were full of incompetent toadies, appointed for political reasons.
Winter War has shown the abilities of these new forces in full colors.
It wasn’t a Soviet air force that toasted Nazy in Spain and army that kicked the crap out of Japanese just a few years ago.
The problem is that period of their stagnation was also a period of major technological leap in both aircraft and tank design. Soviets simply slept through it.
I don’t know if 80% of tanks at battle of Moscow were of foreign make but there were a lot for sure.
The Soviets had only 1700 tanks total after the first Nazi offensive that destroyed 10,000 tanks (!!), of which 670 were at Moscow and the arrival of 466 British tanks was a dramatic increase. Although Soviet factories did recover some, most of those new tanks were not at Moscow.
In general, also see our "Patriot's History of the Modern World, volume 1, chapter 5.