Disclaimer: I wrote that post.
SE allows arbitrary hot-linking of images. The default Imgur upload is there to cover the usual case that the image is not yet hosted anywhere, but there are other benefits as well. Image links are for instance less prone of dying (you can even re-upload images with a link from the web for this reason). A reason for not using it may for instance be technical limitations. In my case, it was that Imgur does not support the svg image format.
Can an svg contain executable javscript?
No. the img tag is sandboxed (for the record, the graphic does not contain JS).
Can I collect your IP adress?
Yes. I can do that with a PNG or GIF too. Whatever resource the site is serving requires your machine to contact the server.
Animations are normally either GIFs hosted by imgur, or YouTube links
Yes they are. Why did I not use that? Because of the effort required. The svg animation was something I had in stock. If you can point me to a tool for converting declarative animations to gifs, I would be very grateful.
My current knowledge of animations is limited to SVG+SMIL and stop motion by piping frames into ffmpeg. The later would require a complete rewrite.
Then: The "Hey, look at my cool website!" aspects of this. This is my only file hosting for uploading arbitrary files. The name of the site is not mentioned in the post, and the link points directly to the file, not to some other page with other content.
Why link at all? Because svg animations go one way by default. Pointer events like the reset button are stripped by browser sandboxing, so to view the animation again the file must be viewed at its location.
Why the post at all? It could essentially be stripped down to just the title. The rest is my own attempt at solving it. My geometry knowledge in this field is a little shaky, so I was not sure about several of the steps (is an ellipse always a circle scaled along an axis?).
Still, looking at it now, it is rather low quality and I deleted it. I should really go to bed.