>Both the USSR and US fought multiple wars directly against the proxies of the other.

They did, but didn't directly go for war between them or raised it to a 3rd world war, which is the point.

>In most any other era one of those would have become a full blown war between the two.

Would it? French and British empires were quite content not to go to war between them after Napoleon, when they felt they had their hands full with their own colonies each. After 1815 they've didn't do it, and even were on the same side in wwi and wwii.