The attackers were not relying only on instant freeze, I guess... travelling tens of kilometers through the frozen taiga, if they were so professional as you say. If 1 hour was needed to freeze the hikers to death, or 3 hours, or 6, it would not make any difference for the attackers.
The attackers may very well have been in a hurry.
The indigenous Mansi people were in the area, and the killing squad as well as the orchestrators above knew that. The longer they stayed, the greater the risk of being observed by the Mansi.
As it turned out, the Mansi were told in no uncertain terms what would happen should they ever tell anyone.
First; some of the Mansi were interrogated and they were informed that they were under suspicion. Needless to say, to be under suspicion under these circumstances meant mortal danger. After a while, the authorities found a seamstress and let her state that the tent was cut from inside, and the story was fabricated that the nine students had cut their way through the tent. Of course, there was no scientific examination, no statement of proof that the tent was cut from the inside. In other words; no proof, just unsubstantiated statement from a seamstress.
What exactly was the purpose behind that sequence of events?
More than likely it was this: The Soviet authorities realized that the Mansi must have seen the corpses after the fact, and of course also the tracks left by the attackers. The Mansi might not know the precise identity of the killers, but they of course understood that the attackers were no common criminals but rather professional special operations forces from a state agency. The Mansi needed to be silenced, but to exterminate whole villages would be too drastic even in the Soviet Union and such mass killing was also not necessary when the silence of the Mansi could be ensured in a much more sophisticated way.
The authorities simply let the Mansi know that they were under suspicion, which must have instilled great fear in their communities. That was exactly the purpose. Thereafter, the same authorities made that seamstress state that the tent was cut by the students. The Mansi were freed.
However, the message to the Mansi was very clear, unspoken and brilliant in its brutal cleverness: "We let you off the hook now, but be aware that if you ever speak about what you saw we will invent any evidence necessary to make you pay dearly. We want you to stay silent."
This is in my opinion a very probable explanation of the mysterious circumstances around the tent, why the seamstress made her unscientific statement, and also why the tent disappeared forever.