I think its a possible future, an one the series may end up going more towards, or at least have the fringe team aiming to to prevent that future. It was defo an odd episode though. I'm sure it ended in a way that would leave you assuming the story will continue further down the line. So it would be strange if it was just the one off episode an it was the end of that story. Season 5 might end up portraying the events that lead up to that future where the team are encased in amber etc.
Or maybe its a glimpse of what season 5 will primarily be about an the plot/story line that season will follow. Either way it leaves a lot of options for them to explore in season 5 an could possibly take the story in a new direction.
The fact Olivia is not in that episode an was killed by William Bell could also signal that maybe shes left the show an will not be in the next season (pure speculation), just seems odd to kill off one of the main characters just like that, an rather randomly too. Doesn't mean to say the other Olivia isn't still alive though, we never saw her or the other side at all.