The license on the code doesn't include trademarks on the name, or copyright on marketing materials, branding, etc. Depending on the project, the bulk of the value might actually be in that stuff and not in the code itself. Don't underestimate how valuable "brand recognition" is.

Also, and while it might not apply in this specific case, a startup / side-project that has paying customers will have value in terms of acquiring the accounts and the relationships, even if the code is open source.

Quote from the article: "Asterank is almost entirely open source." That "almost" might be important.

They might see potential value in a commercial fork aimed at Enterprise or Business clients. There is also the potential yearly service contracts that may be negotiated. Getting the original developer of a program to maintain it is a good idea for Enterprise clients. They can also develop a training package and sell it with his help.

I'd guess that the real value is in the data. They've presumably gathered a bunch of size/composition/value data about asteroids that would almost certainly have value to a company that plans to exploit said asteroids.

I've had an (Apache licensed) open source project acquired. In my case it was the userbase that was valuable.

My bet is: talent + reputation + data.