This was considered an exporatory well. And the “completion” was not to “complete” it for production, it was to be cemented in to “plug” it, but with the ability to drill through the plugs later on.
They thought the cement “plug” at depth (near the pay zone) was doing the job of plugging the hole. Although the driller argued that much of their data said the cement wasn’t holding. A “cement bond log” using a downhole tool was not run. I believe that an earlier attempt at logging the deeper portion of the hole was stalled because the too got stuck in the hole due to casing problems. (Hmmm - there’s a clue things might not be so good down there).
Then they rushed the removal of the heavy mud before the cement would have made a hard bond even if it had been doing the job.
The plan was to remove the mud (replacing with sea water) above the lower cement plug, and then place a shallower plug - I suppose from the ocean floor and down for a certain depth.
At least that is what I have been able to gather from the various articles, and some limited knowledge of how the drilling process goes.
I swear when I read your post, I thought I was reading my own writing.
They did not recognize that gas was already coming up with the mud....that the bottom plug had not worked.
Why is a big question.