"Not prohibited" does not equal an endorsement for a minority to IMPOSE by obstruction an extra-constitutional requirement for confirmation. The Constitution specifies limited items that require a supermajority. Anything else can be passed with a simple majority.
You are correct that not required does not equal prohibited. So if it is the will of the congress and the people to impose a supermajority requirement on confirmation of presidential appointees, then the congress could propose and pass legislation (or more effectively a Constitutional amendment) to impose this new requirement and have it signed by the president (in the case of legislation) or ratified by 3/4 of the states' legislatures (in the case of an amendment). But without such OFFICIAL imposition of a supermajority reqirement for confirmation of appointees, the use of the filibuster is CLEARLY a case of a minority in the senate imposing an extra-constitutional supermajority requirement. There is absolutely NOTHING in the Constitution that would endorse the idea of the minority imposing new supermajority requirements upon the majority as the democrats in the Senate are doing.
True. But there is explicit endorsement of the idea that the Senate gets to make its own rules--"Each House may determine the Rules of its Proceedings." Text trumps non-text, imo. Bottom line, as much as the filibusters suck, I just don't buy the (author's) argument that they're unconstitutional.