Eltima used phrases like “Based on the standard Macromedia Flash player” and such, which makes me suspect they wrote a native-code shell which invokes the Adobe Flash Player in its Netscape Plugin wrapper. The “ExternalInterface” API can then be used for message-passing (rewind, etc). Some clearer docs would help, but if they’re asking consumers to move Adobe Flash Player to their app’s “Netscape Plugins” folder, then there’s no degradation of audience capabilities.

Gnash is a different case, because they’re explicitly writing their own SWF renderer. Recently their home page has changed to say Adobe Flash Player is a security risk because it can make network calls like XmlHttpRequest does in browsers, and I think there’s something wrong with advertising their product like that. But I understood a lot more when I saw Rob Savoye speak — it filled in the background for me of how he thinks:
http://tech.netscape.com/story/2006/10/03/oreilly-eurooscon-rob-savoye-of-gnash/

The last thing we need is to worry about non-standard versions of Flash players. One of the things that drew me to developing for Flash is the neglible need to worry about the player acting differently across various user environments.