I believed the same; it never seemed plausible to me that Amazon actually thought they could make drone delivery profitable. But... organizing an 850-person division which builds actual hardware sounds more like a real investment than just a PR effort.
A big part of the PR is internal - to make people at Amazon from the top to the bottom feel like they're still working at a "Day One" innovative company building cool new tech that they can excitedly tell their family about over Thanksgiving dinner. That's why it has to be a bit bigger than just some AI-generated stock imagery.