both seem to be about fast scalable olap analytics, but what is the difference?

Kudu is just a storage engine for fast analytics on fast data. That means you can write data to Kudu in realtime, then query it using the Spark, Impala, Presto etc. Pinot on the other hand is a realtime OLAP datastore includes storage and query engines with low latency. You can compare Kudu with parquet, ORC or another formats but pinot is very different than others.

Thanks @User. Also, to add Pinot can consume data directly from stream systems (eg Kafka) as well as offline, and provide near-realtime analytics for user facing analytics at high throughput (10's of thousands of reads at p99 read latency within tens of millis).

hmm i wonder why then we have apache druid, impala, and apache drill.. i understand it's slack is for Pinot strictly, but still - why create so many seemingly similar analytics platforms?

You can refer to this thread: https://apache-pinot.slack.com/archives/CDRCA57FC/p1619110391467000