Vocabulary please. A "codec" is software that CODes and DECodes multimedia content, while specs describe an encoded file or stream format (occasionally involving network protocols and other concerns).

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.

The way to criticize that comment is to point out that all the major and most important codecs that are most commonly used with ffmpeg, do not come from the ffmpeg project. H.264, H.265, libmp3lame, speex, libfdkaac, etc. all come from other projects. What ffmpeg does is provide libraries for transforming decoded data between formats and calling to and from encoders and decoders and multiplexers and bitstream formats.

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.