How did they blur the backgrond here? A gaussian with a bit of motion blur? Custom kernel?

https://mastodon.social/@anicasts/112810812949935074

what interests me is that they also blurred the edges of the foreground characters as well, which seems like addtional work for minimal gain
@mary Now I'm curious about what wild and wonderful composition effects people have implemented in whatever digital composition tools anime studios use to put everything together. At least I think things like blur are done in the final composition step, instead of handing the composition people pre-blurred elements.
@mary radial blur with chromatic aberration and gaussian blur i assume?
@natty yeah i did think initially think there was some chromatic abberation there but wasn't sure if i was misinterpreting motion blur