A word of caution.
I've tested this a bit further and this is most likely a hack.
It works only if you give the post type the same name as the section where you want this post to appear.
Meaning set the frontmatter
For section posts
Perhaps Hugo's gurus like @bep or @rdwatters have a better way to point to the same content file from different sections, than what I posted above.
Interesting. It seems that posts already have their type set according to the section they originally belong to. Meaning a piece of content under /posts/ already has
type = "posts" even if it's not explicitly declared in the frontmatter.