It's disappointing to see. It doesn't take much work to configure a MQTT server to require client certificates for all connections. It does require an extra step in provisioning to give each device a client certificate. But for a commercial product, it's inexcusably negligent.
Then there's hardening your peripheral and central device/app against the kinds of spoofing attacks that are described in this blog post.
If your peripheral and central device can securely [0] store key material, then (in addition to the standard security features that come with the Bluetooth protocol) one may implement mutual authentication between the central and peripheral devices and, optionally, encryption of the data that is transmitted across that connection.
Then, as long as your peripheral and central devices are programmed to only ever respond when presented with signatures that can be verified by a trusted public key, the spoofing and probing demonstrated here simply won't work (unless somebody reverse engineers the app running on the central device to change its behaviour after the signature verification has been performed).
To protect against that, you'd have to introduce server-mediated authorisation. On Android, that would require things like the Play Integrity API and app signatures. Then, if the server verifies that the instance of the app running on the central device is unmodified, it can issue a token that the central device can send to the peripheral for verification in addition to the signatures from the previous step.
Alternatively, you could also have the server generate the actual command frames that the central device sends to the peripheral. The server would provide the raw command frame and the command frame signed with its own key, which can be verified by the peripheral.
I guess I got a bit carried away here. Certainly, not every peripheral needs that level of security. But, into which category this device falls, I'm not sure. On the one hand, it's not a security device, like an electronic door lock. And on the other hand, it's a very personal peripheral with some unusual capabilities like the electrical muscle stimulation gizmo and the room occupancy sensor.
[0]: Like with the Android KeyStore and whichever HSMs are used in microcontrollers, so that keys can't be extracted by just dumping strings from a binary.