I don’t think it is used to mean literally by law. It means that the practice (being critical of other languages) was formally or explicitly encouraged by the community, rather than being an unspoken but common practice.

De jure is almost exclusively used when there's a contrast between theory and practice. "The UN seat is still held by the de jure government of Afghanistan, but de facto, the Taliban now have full control of the country."

Yes. The writer was clearly referring to the practices which were explicitly condoned by that organization. The organization was not literally a government, but the term still makes perfect sense when referring to the governance of an organization. The writer’s point was that being critical of other languages was not simply something that happened in practice, but was an explicitly condoned behavior.

No, they were saying the practice is demanded by custom, which is precisely what de rigueur means.

There is no organization of snobby Pythonistas etc mandating that its members make fun of PHP, and even if there was, de jure wouldn't make any sense. Quite the opposite, in fact, since it was de facto required to be critical of PHP.