This is quite different from Chernobyl, which didn't even have a proper confinement wall. It's unlikely that it can get as bad as Chernobyl was.
This is just built-up decay heat and chemical reactions.
Based on the description of two-stage containment for BWRs at wikipedia[2], and the video, if that's the containment strategy at Fukushima, it seems the drywell containment structure blew up, since steam being vented into a pool of water shouldn't cause a visible breach.
[1] http://e.nikkei.com/e/fr/tnks/Nni20110312D12JF520.htm
[2] http://en.wikipedia.org/wiki/Containment_building#Boiling_wa...
The containment referred to in this case is a vessel made of a feet-thick layer of concrete surrounding the reactor. This is the final defense if a fuel meltdown occurs: its job is to contain the burning fuel and prevent it from being exposed to the environment. [1]
What the GP was referring to is that there was no such vessel at Chernobyl, which was a very old reactor design. Modern reactors are surrounded in such vessels. [2]
[1] http://en.wikipedia.org/wiki/Containment_building
[2] Here's some information about the reactor design at Chernobyl: http://en.wikipedia.org/wiki/RBMK#Containment
Another interesting read is the story of the Russian engineers who prevented an explosion at Chernobyl by diving into the pool of water beneath the reactor and opening the drainage valves: http://en.wikipedia.org/wiki/Chernobyl_disaster#Steam_explos...
But if the outer building which is supposed to contain everything else blows up - that's not exactly the best case is it?
You could think about the outer building as keeping the rain out, and possibly aeroplanes to some extent. It is not the critical last defence against stuff happening in the reactor, the containment building is. You can bet that the japanese army is keeping any possible threats out of there.
I also read that on TMI a similar hydrogen explosion occurred but it was contained by containment dome.