Because ingestion time and processing time are basically the same. It's wall clock.

We did document it in the past, see https://nightlies.apache.org/flink/flink-docs-release-1.11/dev/table/streaming/time_attributes.html

But it's not a correct representation

when should we use ingestion time and when processing time?

This ultimately depends on your requirements and nothing more 🙂

This is the populair explanation that’s used

So event time that’s coming in from the data which is designated as the timestamp that’s relevant for this data, e.g. “This is when X happened” and processing time is “This is when data X has been analyzed”

I think it's a semantic discussion in the end: what do you define as ingestion time? Is that the moment let's say an event from Kafka is read? But what does that mean for the next operator, for example to aggregate the results? How is that different from processing time?