It never combusted. The permafrost melted and it all just went in the atmosphere and the loss of mass caused a sinkhole. The summary is bad. There was never a explosion besides the dust settling.
You're right, the summary could have been better, however I can say in its defence that the summary as you read it now is an awful lot better than the one I submitted.
It's quite a reasonable assumption that combustion is involved when coming across a discussion about a methane explosion, however in this situation there was no actual combustion. Despite this, the event was still quite accurately described as a methane exploding. Is there a common method of differentiating the two other than affixing a disclaimer about a "pressure explosion" versus a "combustion pressure explosion"?