I agree charity is not the right way to frame it. But if this was a pure money decision they would save time and effort forking the language and improving it as

I agree charity is not the right way to frame it. But if this was a pure money decision they would save time and effort forking the language and improving it as closed source. They have the expertise to run a language fork and because of the scale they run at they would easily save more money doing so. Meta contributing to OSS is not an economic decision. It is about branding, trying to attract technically strong individuals (who tend to contribute to OSS or want to), and sticking to their open culture.