Hello
For the convenience of other readers, I'll paste what is written in the URL you referred.
common error ~ that/which
If you are defining something by distinguishing it from a larger class of which it is a member, use "that": "I chose the lettuce that had the fewest wilted leaves." When the general class is not being limited or defined in some way, then "which" is appropriate: "He made an iceberg Caesar salad, which didn’t taste quite right." Note that "which" is normally preceded by a comma, but "that" is not.
What they say here is that we should use only 'which' when the relative clause is used as a restrictive adjectival clause. It is exactly what we ESL students are taught in school. We are also taught we have to put a comma before 'which' when it is used as a restrictive relative.
As for your sentence, it is somewhat difficult to decide whether "that" is used as a restrictive relative or not. But I believe the writer put a comma before ''that' for the purpose to punctuate the inserted phrase 'including decisions of the court'. So I rather take the sentence as "Justice Ginsburg discussed how there have been many federal court decisions that have reached the merits of third-party constitutional challenges to tax benefits without mentioning the TIA". If we change it to "Justice Ginsburg discussed how there have been many federal court decisions, which have reached the merits of third-party constitutional challenges to tax benefits without mentioning the TIA", it could sound nonsense. It is no surprise that there had been many federal court decisions and it would not be anything so worthy as Justice Ginsburg should discuss.
paco