Usually, time series databases use a variety of tricks to store data using as fewer bits per sample as possible. An OLAP database sounds like a much more generic system that will cover a larger number of use cases, but won't be finely tuned resource-wise for this one aspect.
Can I read about how the time series nature of the data allows the storage engines to optimize different parts of the system compared to generic databases? Some links? Papers? Interested to know.