At my job we use Quip strictly for collaborative design docs (i.e. architectural design, project scope, tutorials etc). Personally I don't think it can be a true wiki platform since it lacks robust indexing -- this is without doubt its biggest shortcoming.
Correct. Quip is not a wiki. I would call it an “opinionated and stripped-down version of GSuite with excellent collaboration support”. Quip also has chat stapled to it.
Slack is missing doc management/creation. Quip is missing a good chat solution. It’s almost like PB&J.