Not quite; they estimate that the battery will hold at least 24 hours of charge after 15 years. Charging it back up takes one hour, and in the context of cochlear implants these are very good numbers. I think most people would be okay with a (let's say) 20-year reimplant cadence given the benefits of this technology - but this obviously depends on which country you happen to live in. I'm lucky enough to live in a place where cochlear implants (and the surgery) is free to whoever needs it.

They didn't test this in my house, that's certain.

I kinda view Lithium batteries as road flares with good PR. So that inside my body next to my brain seems..sketchy.

The other issue is that cutting it out and replacing it is the only option when the battery dies or goes wrong. If the product/company dies in the meantime, you have a bunch of scar tissue and no hearing. Replacing the "normally inside" bits of a cochlear implant is really high risk of not working as intended, so changing brands isn't a small ask. You really get to do this once. So putting the processor inside and tightly integrated with the nerve stimulation seems inadvisable. At least with the processor separated, you have a chance a compatible replacement processor could be made and exchanged without huge risks.