Starlink have over 11,000 satellites in orbit.

Globalstar have 24 satellites in orbit (at least according to Google).

How does this acquisition by Amazon meaningfully expand Leo to compete against Starlink?

Are those the iridium satellites? They are doing something completely different from starlink. For one thing, they are in much higher orbit, increasing both latency and coverage. They also have much less available bandwidth.

I'm not sure they are competitors. They could be, but these networks are serving different purposes.

Maybe spectrum.

Spectrum and IP, probably.

Pre-existing infra, designs and know-how?