Well, seems to be a moot point. Since I have a 20-inch, decidedly non-4k TV, I went with the 2d-generation Fire TV Stick, which is also (nominally) supported by Tidal, in order to save a couple of bucks. So: plugged the FireStick into my Marantz 1607 (Atmos-capable, though it tops out at 5.1.2), installed the FireStick, installed the Tidal app within FireStick, checked for and installed a FireStick system upgrade, loaded some Atmos content in Tidal...and it only plays in stereo.
According to Amazon, 2d-gen Firestick's latest OS is 5.2.7.3, not 6.2.7.1, so maybe that's the issue. But if that's the case, then why list the whole 2d-gen "family" of Fire TV products as supported devices if they're not?
Not sure I have the fortitude to type out support tickets to both Tidal and Amazon and slog through all the back-and-forth. If there was phone support, and it was staffed by knowledgeable people, then maybe. Otherwise I think my early-adoption adventure ends early...