But since Loki is many times used in observability use cases, where there is sometimes a need to query high cardinality data, we thought to include it.
The difference is that in Loki the index is only used for metadata around the source of the log lines (environment, team, cluster, host, pod etc) for selecting the right log stream to search in.
Parsing, aggregation and/or filtering of log lines on high cardinality data is all done at query time using LogQL. See also https://www.youtube.com/watch?v=UiiZ463lcVA and this live example where a 95th quantile is calculated using the request_time field of nginx logs https://play.grafana.org/d/T512JVH7z/loki-nginx-service-mesh...