The designer has created an animation in After Effects which has to play at some location on the web page. I can have the designer export a video and insert it that way, or I can convert the AE file to a Lotte animation. It looks the same to the user, so it's ultimately just a matter of bandwidth.
Are you sure Lottie + SDK is heavier than the mp4? For one or two animations that hasn't been my experience, especially if you can do WebM and fall back to optimized mp4.
This was at least six years ago. At the time, yes I'm pretty sure Lottie was a lot smaller. However, iirc we largely abandoned it in favor of videos anyway because the lottie animations wouldn't render correctly without additional work.
I doubt the designer did any sort of compression on that video. Animations usually compress very well.
The less-abridged process was "the designer exports a ProRes video, and I (personally) spend an hour experimenting with different ffmpeg settings to get it as small as possible while retaining a level of quality the designers will accept."