When I compared the versions of that sutra, it became apparent that a large number of the items in DN are straight out of the version of the Sarvastivadins, almost verbatim. The same material isn’t included in the other versions like the one in DA (which is much smaller). It’s the kind of thing that isn’t obvious until a person sits down and does the hard work of comparing a bunch of parallels. Generally speaking, the DN Sangitisutta looks like an amalgamation of material from several other versions. It was eye opening to me; I stopped thinking of Theravada as being so sectarian.
[Edit: For those who are interested in more detail about this, here’s the thread I created on the topic when I was translating the DA version: An Example of EBT Expansion: Comparing Four Versions of the Saṅgīti Sutta (DN 33) I never wrote it up in greater detail because … it’s a huge project. ]