This article is about audio recordings. There's no mention of Bluetooth nor any mention as to if there were any relevant recordings, which as I understand it are not stored on the device at all.
This smells like an urban myth.
I don't think it's implausible that an Echo would have an internal list of trusted Bluetooth devices and their last date of connectivity.
I don't think it's implausible that an Echo would have an internal list of trusted Bluetooth devices and their last date of connectivity.