They either built a snow cave or not, regardless, to me it seems like even if it collapses on them, even 4 meters of snow, it cannot break Tibo's skull.
But what we know is that somehow they ended up under 4 meters of snow... and in a stream. Now, it couldn't have been 4 metres deep when they went there, because then they would sink into it. I would think maybe they can enter the stream elsewhere and walk along it to a place where there is high snow on its banks and build a snow cave but... do you really want to get your clothes wet when it's -20 ~ -30C? I say no.
So how did they end up there. In my mind the only possibility is that the snow shifted, the stream melting it from below and new snow falling on top. So naturally, anything in the snow ends up in the stream, because it melts from below you and melts faster just above the stream. This doesn't tell us whether that thing (or person) was originally on top of the snow or embedded within it. Both are possibilities. Maybe there was no snow cave at all. Maybe the "flooring" made out of branches was a wind shelter originally propped against a tree, and the wind blew it away and then snowfall covered it.