Not necessarily. Logstash (as used in our fully worked out example) is a very useful ETL (extract, transform, load) tool for loading data into Elasticsearch and the Siren platform, but it is not the only choice.
ETL can be performed in many ways, for example by using custom scripts, or by using ETL/data workflow/messaging tools such as Talend, CloverETL, Kafka and so on.
Also consider that in Siren you don't necessarily have to "load" your data, but instead you can use the federation/virtualization capabilities to directly connect to a JDBC backend (see the documentation for datasources/virtual indexes). Lastly, in 10.2 we have introduced easy ways to upload data via CSV/Excels.