"Whatever. Post-Founding rules said no spirits allowed, so she decided not to overthink it."
This line refer to an "anti-superstition" type of policy, like the ones implemented in China after 1949 under the new government. Around the early 1950s, the Communist Party issued strict regulations against superstition, folk religion, and anything considered "feudal" or "spiritual." The goal was to promote scientific thinking and suppress beliefs in ghosts, spirits, or other supernatural entities. Temples were often closed or repurposed, mediums and spirit worship were banned, and things like divination, spirit mediums, or claiming to have magical powers were officially outlawed.
In the story, the line "Post-Founding rules said no spirits allowed" is a nod to that historical background—it's basically the world acknowledging that, according to official regulations, "spirits" or supernatural interference can't exist or at least shouldn't be referenced openly.
Jing Shu, chooses not to overthink it because it's both a legal/societal constraint and a narrative way to explain why she doesn't treat certain spiritual phenomena too seriously.
---
I pluck this from my footnote on chapter: Others Bring Dogs, I Bring a Hen, The Cube Queen's Apocalypse Feast, Volume 4: The Quaking Harvest