At the time they were doing functioning mockups for the Aqua design in Macromedia Director. It was not easy to actually implement the UI with the available hardware, but the mockups showed that it was possible. "Mr. Ratzlaff frequently would remind Apple engineers that 'Macromedia can do it, so why can't you?'"

Only source I can find is: https://www.macobserver.com/article/2006/03/15.9.shtml

> As it so happened, Mr. Ratzlaff and his team had already been working on a ground-up redesign of the operating system after they discovered that their original goal of porting the look-and-feel of Mac OS 8 completely to Mac OS X would be impossible. "We were only going to be able to get about 95% of the way there [putting the Mac OS 8 interface on Mac OS X], which is possibly the worst thing you could do," Mr. Ratzlaff said. The redesign had been scrubbed by higher-ups at Apple but was put back on track by Mr. Jobs.

> With that, Mr. Ratzlaff and his team's list of requirements for the operating system, which had been called overly ambitious and led to laughter from engineers who heard of them only weeks prior, became mandatory. These included 32-bit color with alpha channels and QuickTime integration, all being able to run on a system with just a G3 processor and as a little as 8MB of video memory. Mocking up the operating system's design and functionality with Macromedia's Director, Mr. Ratzlaff frequently would remind Apple engineers that "Macromedia can do it, so why can't you?"