I'm afraid the article is also wrong, this is a typical misconception when working with Kafka. Offsets are committed in the next poll() invocation. If the previous messages weren't processed, a rebalance occurs and messages are processed by other instance. This is an implementation detail of the Java client library but it allows the at-least-once semantic with auto-commit. The book Effective Kafka has a better explanation.

librdkafka isn't part of official Kafka so it may have problems with this as it has other limitations.

In any case, the report isn't right about this and it doesn't use the safest options. Commit offsets manually is the most flexible way but it isn't easy, being the error more usual to commit offsets individually