Well the Soviet team did not have 90% of their best players at the Canada Cup in 1976 and 1991.
Comparing the 1976 Canada Cup roster to the Soviets gold-medal roster of that year's Olympics, one does see some big names missing, but not 90%.
On defense they were without Gennady Tsygankov and Yuri Liapkin, while at forward they were missing Vladimir Petrov, Boris Mikhailov, Vladimir Shadrin, Alexander Yakushev and Valeri Kharlamov (though he was out to due to car accident injury).
Serious losses, no doubt. Yet not that much worse than Canada being without Patrick Roy, Mario Lemieux, Paul Kariya, Ron Francis, Al MacInnis and Ray Bourque in 1996. In fact I'd say Canada's losses in 96 were worse. Best goalie and Stanley cup MVP of the previous season (the Soviets at least had Tretiak in 76), the top three Canadians in scoring the previous year, and two of the top defencemen.
It's in 1991 were Soviet losses were more serious, mostly due to defections and/or players not being invited.
Goal: Arthur Irbe (refused to play), Sergei Mylnikov
Defence: Vyacheslav Fetisov, Vladimir Konstantinov
Forward: Igor Larionov, Vladimir Krutov, Sergei Makarov, Alexei Kovalev, Valeri Kamensky (injured in exhibition game), Pavel Bure, Alexander Mogilny, Vyacheslav Bykov, Sergei Nemchinov, Andrei Khomutov, Dmitri Khristich
They definitely wouldn't have finished fifth had they played at full strength.
Ironically, five years later at the first World Cup, Russia sent their strongest team in years, one that was missing only Vladimir Konstantinov (injury), German Titov (decllined), Valery Kamensky (no NHL contract), and Pavel Bure (injury). They finished a disappointing fourth.
Two years later in Nagano they sent a team that was almost as depleted by no-shows as 1991 and got the silver. Go figure!