Does anyone know why this chibi from Kaiju 8 wasn't added?

idk Mal has always been picky about what they let into their database. some good examples of stuff not being here are:

Scott Pilgrim takes off,
Ricky and Morty the anime,
lord of the rings war of the rohirrim,
Dragons of Wonderhatch.

I would say somehow bug them about it until they add it.

they might add it eventually

Here's the entry https://myanimelist.net/anime/60480 Which is still pending approval.
Normally, database mods don't automatically add these kinds of short or special episodes. Users need to submit them and wait for approval, which mods also usually overlook.