My understanding is that HyperLogLog will estimate the cardinality e.g., the number of different types of events in the stream. The frequency counting algorithms in the article can be used to estimate the number of occurrences of each of the most frequently occurring types.

E.g., Where the frequency counting algorithms might estimate the following counts for a given stream: A: 2, B: 5, C:1 ...HyperLogLog would estimate that you have three types of events (A, B and C but you only get the count: 3) in it. Presumably, HyperLogLog requires fewer time and space resources to achieve this more summarized perspective.