Ah, interesting observation. So, what used to happen was the old DOS-based ADT would be built and included in the "disks" directory. It effectively mirrored what was in the official ADT repository (used to be Berlios.de for those with a long memory), but had some settings tweaked to be more in line with ADTPro (faster default speed comes to mind). Over time, the more-or-less official repository for DOS ADT moved to GitHub (https://github.com/david-schmidt/adt) and while the code is still shared and built, ADTPro doesn't add the disk in its distribution any more. So that's what exactly is meant by "integrated." It's partly a holdover from a different time, and it's partly an oversight for not bundling the disk (which is no longer strictly needed).
The reason DOS ADT is relevant at all is because it runs in 48k (where ADTPro does not), and there are some legacy servers for DOS ADT that go where ADTPro server can't (MacOS 8.x and earlier, PC DOS, etc.) I used to say speed was DOS ADT's advantage too chiefly because it turns the drive motor on preemptively; but now ADTPro's client does that too, and the tunable payload size gives ADTPro a much greater speed advantage.
I just remembered, yes, you can push EsDOS as well as the DOS ADT client via serial bootstrapping operations. So while the actual disk doesn't get included in the distribution, the virtual image of the program itself is available to push over the wire. That version is 2.41.