It's a land grab that, if GSAP becomes a standard, makes it that much harder to compete with Webflow.
For example GSAP will now never make its way into Figma or Framer which is a staple of the design workflows of many teams. This move by Webflow makes GSAP a line of demarkation between Figma-based workflows, Framer-based workflows, and Webflow-based workflows.
Any momentum the library might have had is now skewered by these limitations which will surely create demand for more different animation libraries to exist within other walled gardens.
The same happened to Vercel & Next.js which now seems to be powering the return of Vite & CSR.
Now that said, I'd never heard of this library until this announcement, and I don't know much about its role in the javascript ecosystem, but I can't say that the trend towards M&A kingdoms in the open source community delights me.
FWIW, sub-5kb banners were probably the only Flash projects that didn't include GSAP before we collectively jumped ships to JavaScript and CSS for animation on the web. It was everywhere.