Particularly in the latter cases, most of these tools embed the machine readable diagram inside of the PNG. So you can just open the PNG inside of draw.io or excalidraw (if it was created by that tool) and edit the diagram. This is because the plain text file is embedded.
It’s great that lots of tools are going this way. Notably, if you’re doing image generation with Stable Diffusion or Flux, you can just drag a PNG from ComfyUI into the ComfyUI window and get the same whole workflow. It’s a great learning tool.
As for diffs? Well, that’s hard - even if you were diffing the “source” text files for excalidraw or plantUML, a lot can change that makes it nearly impossible to understand. When all that really happened might’ve been a small styling change and moving a node.