Unbelievably stupid indeed
Wait, it gets worse!
I have two phantom devices, one that appears in the iCloud Drive prompt (I still haven't accepted the migration offer, as I'm waiting for Yosemite GM), and another in the “My Devices” section…
Both were MacBooks from friends of mine, both named Joana (my name is João, btw; both names are portuguese versions of Joan and John), and the former was booted from Mavericks and the latter was booted from a Yosemite Public Beta installation I've been using on my iMac (which appears only once, even though it is booted regularly from either Yosemite PB and Mavericks). The first Joana's MacBook appears as “MacBook Pro de Joana” (literally, “Joana's Macbook Pro”) in the iCloud Drive warning field as one of the devices that must be upgraded to Yosemite, and the second appears as “iMac de João” (“João's iMac”; I may have customized the former's “computer name” and not the latter's, which explains why it retained the iMac's external Yosemite PB boot drive Sharing preferences) on the “My Devices” panel. Weirdly, the MacBook running Mavericks doesn't appear on said panel…
So this is definitely a Mavericks/Yosemite/iOS 8/iCloud Drive-related issue, it seems, and an incoherent one at that. Once you boot the sucker and log it in to iCloud, it automagically and irreversibly reports the device's serial number to Apple/iCloud; Mavericks only seems to report it to the iCloud Drive compatibility check (which is nice, but should be reverted from the moment the user *logs out* of iCloud, which I obviously did), while Yosemite forcefully registers it in the “My Devices” section.
I'm guessing the manual “support profile” registration *and* the iTunes “trusted devices” list weren't enough for Apple, eh? :\ Now we all have three device lists, all out of sync, and one of which is auto-updated and un-customizable!
Add to this the iCloud data loss bug, and suddenly the whole Maps debacle starts to look like a non-issue by comparison.