I think the point they’re making is you can effectively have a self hosted non federated option with Matrix. Just disable federation as a whole (which I’m pretty sure is completely possible. Given companies use matrix for comms, and might not want federation, for similar reasons to what is being discussed here)

Don’t have a good rec to add to the discussion of hardware, but I would love to give a shoutout to GadgetBridge, which is an Open Source Android app that is a vendorless alternative to controlling lots of smart wearables (and other gadgets). It doesn’t currently have Polar support, but seems that might be fairly easy to implement, since polar publishes their SDK online (and presumably the protocol with which it communicates).