There's a roughly ~12,000-word postmortem document that took an additional 2+ months to produce after the fact, which the OP has lightly quoted or summarized from in his post (including what you quoted). It really is an impressively magnificent failure at scale, but there's nothing really fundamentally earth-shattering about how this project went so far off the rails. Mistakes were made, but IMO nobody was outrageously negligent.

The OP's assertion that it was primarily due to slavish devotion to InfoSec's insistence of getting off PHP-based MediaWiki, is incomplete and misleading. The team itself opted into the migration, and by their own admission in their own postmortem document, they wanted the opportunity to get off MediaWiki (killing two birds with one stone, both as a technology upgrade and as an InfoSec compliance thing), but they fundamentally vastly underappreciated the scale and complexity of the project (some top line #'s: ~4M total documents, 1.7M unique pages visited daily, 600K unique MAU, etc.). What probably doesn't come through in talking about this, is that the wiki is far from just a simple collection of text-based wiki pages in the standard sense; it serves more like something akin to an unholy abomination of a Wordpress-like platform with endless amounts of plugins and macros and templates, and gives you just enough programmability to be dangerous.

The primary reason it took 4+ years, was essentially down to going through multiple rounds of failed migration attempts, including failed attempts at producing automatic translations of wiki's from one platform to another (which proved to be incredibly complex, due to the nature of how customizable MediaWiki was and how many teams had gone and done so many interesting/advanced things with it, which was great... except it made automatic translation/migration near-impossible).