I've always seen Bellard as an engineer who programs rather than a pure programmer.
The format for all modern video codecs is not the kind of format where any specific piece of uncompressed input should always be encoded the same way, but more like a very restricted programming language that gives the encoder a lot of tools to compress the video, and which tools they use and how they use them are up to them.
if your mental model is that somebody writes codec specs and then fabrice bellard comes in and turns the specs into C, you are dead wrong. first of all, codecs are usually reverse-engineered, there is no spec. second of all, even when a well specified document describes the codec, that spec does not describe how to efficiently encode or decode with that codec. people like fabrice bellard develop the algorithms that do that.
In a normal standard development process experimental codecs come first, then those that have proved to work well, including having good enough performance, are described in the spec; after standardization there's very little room to "develop the algorithms" because nonconformant implementations would be useless.
Reverse engineering is limited to the abnormal case of having access to some codec but not to the standard that describes it.
It may also be worth pointing out, in terms of apportioning credit fairly, that ffmpeg has not been Bellard's project since 2004. The thing we see today is no more his project than GCC or Emacs are Stallman's projects.
These days, I tend to mix them all together, and I think I get good results.
I strongly suspect that a lot of folks, these days, only do the middle one.
Ain't no one willing to pay for all of that. The clear separation is something you only see remaining in academia and industries where code quality issues have legal consequences (i.e. aerospace, marine, automotive and medical), and even there, pressure is high to relax rules viewed as "arcane".
Writing good specifications, documentations, implementation code and tests each is an art form in itself
That fact that you can (almost) freely mix and match processing between such different worlds is quite an achievement and libav (IMO) is decently well designed to allow that.