The fact that there's draft.js, and a myriad of other existing editor libraries out there already, all of which could've been contributed to instead.

If a startup chose to base their technology on a library that is released by a major corporation, they face the risk of getting "rug pulled" (or the library updates incompatibly and now you're locked in either to using the old version, or painful upgrade to the new version).

Of course, they don't have to choose to use such a library, but if they don't then there's a community of people who then questions them on why they are spending time replicating a technology.

The point is, if a startup chose to use this library, they face the risk of having to expend time keeping up in pace (and associated cost of doing so) with facebook.

This equivalent scenario exists in someone writing against libraries released by microsoft on windows, and that's what spolsky is complaining about.