This is the ADT-3 and unsurprisingly follows on from the ADT-2. What is surprising is the time-frame as the documentation points to the ADT-3 launching in the fourth quarter of 2019.
The reason this is so surprising is due to how new the current ADT device is. Google only announced the ADT-2 in 2018 although it was months before developers started getting their hands on the device. Even now there's still plenty of Android TV app developers who have yet to gain access to the ADT-2.
Compare this to the previous device turnaround and it becomes abundantly clear just how quick this follow up device is arriving as the ADT-1 launched when Android TV launched back in 2014. While it took Google around four years to replace the first model it now seems ready to replace the second model after only one year. That is, if the ADT3 is indeed a direct replacement. The ADT-2 arrived in dongle form and it might be the case this ADT-3 is designed to test on a different device altogether. Traditionally, this would be assumed to be a new set-top box, but it was only this week when Google announced the arrival of "Android TV soundbars" and so it could be a soundbar, or any other product Google thinks it can bundle the Android TV platform with.
Regardless of the design, it now seems highly likely that a device codenamed "archer" is the ADT-3.
Archer only came to light last week and at the time it was unclear what the device was. The device was spotted in conjunction with AOSP and was being used to test various Android TV features along with the Nexus Player. While it was clear it was an Android TV device, and likely a Google device, it seemed highly unlikely it was an ADT device simply due to how new the device was. Although, it remained even more unlikely that it was a new consumer Android TV device by Google, such as a Pixel Player.
Based on the timing of the spotting of Archer and this new information, it stands to reason the ADT-3 and Archer are the same device.
As an interesting side note, the document detailing the ADT-3 information states "Q4'2019 – ADT-3, for App Developers, and official Android TV release." The image of the document seemingly cuts-off the end of the sentence but it stands to reason the message is likely referring to the release of Android Q for Android TV.
The same documents also reiterate that the "P GTVS" window closes on March 31, 2020. GTVS is "Google TV Services" (basically, Android TV) and the "P" is evidently referring to Android 9 Pie. The takeaway here is that all new devices launched after March 31, 2020 will need to be launched already running on Android Q.
Needless to say, Android Q for Android TV will need to be released way ahead of that date for testing to have been completed by both app and device makers. This, along with the original cut-off message does appear to confirm Android Q for Android TV will launch in Q4, 2019 – along with the new Android TV device for developers.