Open standards might have been a better term. The example in this project would be the use of "Pencil Project" for UX design rather than Figma / Sketch / Adobe XD (which to some extent may be for licencing and cost reasons too).
Figma, because of the cost (or lack off) and availability, seems to be the only tool realistically available for doing open source design today. Unfortunately neither the format nor the editor is open source itself, but sometimes the means justify the end I guess.
I doubt the click-through presentation stuff can be exported in any way, but basic design work could be persisted and collaborated on by multiple people with different tools if that approach was used.
Currently for my open source project, we all just work directly in Figma then export the design to a website via the tool we're building. Works fine for now, but not doing anything too complex. Project is https://github.com/instantwebsite in case you're curious.