The seal leaked during initial static pressure testing, it leaked during test-firings, etc. Engineers and management at both Thiokol and NASA knew about this. NASA engineers repeatedly objected to the Thiokol design (both original and modified) for different reasons, talked to the o-ring supplier who stated that the design was using o-rings in a way never used before, etc.
The author of the blog post is wildly wrong, but so are a lot of comments.
Everyone, PLEASE read the Rogers Commission report. It spells out the extensive problems with the design/manufacture, and at both Thiokol and NASA.
https://www.nasa.gov/history/rogersrep/v1ch6.htm
Edit: I can't post a response because my account has a posting limit, but "the o-rings were not defective" is...misleading.
The o-rings were constructed as an assembly that used multiple lengths of o-ring material glued together, instead of the entire o-ring being molded at once, which is what had been done on prior rockets. Up to five joints were allowed. No inspection of the glued joints was performed other than a surface inspection.
Second edit: no, the problem is not that the "selection team did not account for atmospheric pressure." The joints between sections mechanically did not hold together correctly. NASA engineers predicted this when examining the revised design during the earliest phases, although the assembly was found to act in a way different than how they had predicted, but still caused the seals to leak. Everyone knew the seals leaked, before the first shuttle headed to the launch pad.
Third edit: the blog is "wildly wrong" because it claims NASA supplied or modified the revised seal design and outright declares them incompetent government bureaucrats who didn't know how solid rocket motors worked. In fact, both designs came from Thiokol in entirety - and NASA engineers basically said in reports something to the effect of "the government (ie NASA and the military) has never seen a solid rocket motor sealed like this".
When NASA engineers approached the o-ring manufacturing company, the company said they'd never seen a design like it and felt that it was 'not being used like an o-ring' or something to that effect.
From the very beginning NASA engineers were screaming their heads off that the design was shit. Testing validated their concerns. Upper management at both Thiokol and NASA didn't care.