webm seems to be the modern replacement for GIFs in browsers. I would suspect gifs.com supports them. Somehow their site is buggy for me atm so I could not test it.
The extension trick works in all sorts of place. And in reverse too. You can often upload an animated GIF to sites that don't support it by just changing the extension to .jpg or .png.
WebM is just a container format, though it's usually used with VP9 and opus, and is functionally equivalent to using any other kind of actual video.
Animated GIFs remain quite useful for non-video animations where you don't want lossy compression, and want to use other GIF features like transparency. Animated PNGs are a potential replacement, and support all of the features of GIFs along with 24-bit color, but software support remains lackluster.
Playing with the quality options with Gifski can yield a significant drop in size sometimes without much drop in quality (sometimes no noticeable drop in quality unless you really look). Perhaps gifs.com are using the same (or similar) algorithm with more size-optimised defaults?
Wish these services could be run locally. I'd even pay for it. For people at companies uploading a clip of something internal for a slide deck to a 3rd party is a non-starter.
I thought they were using Gifski but I'm not sure about it.