This page claims that Coub used to offer a player you could embed into your own flash content, and that it was deprecated in February 2021:https://coub.com/dev/docs/Player+API%2FFlash+player
It is the only search result in the documentation for "flash" or "swf".
I found this by googling "coub" flash -inurl:/view/https://coub.com/stories/12163-flash-collection
A small collection of coubs that could've been swfs before.
I tried two different downloaders on a meme that's definitely a flash and only got a mp4 file.
It's possible that Coub allowed flashes at some point.
It's possible that they could've all been converted when the time came.
But it's also possible that they were never allowed in the first place, and you could only upload mp4 (or swf that gets converted right away) and music.
I don't know at all.