jena-core contains an old Turtle/N3 writer. (The N3 is for data only - not N3 nested graphs).
Jena-core only needs Turtle/N3 because there are tests written using Turtle.
The jena-core parser and writer are not up-to-date with the Turtle W3C Recommendation and are replaced in normal use by the Turtle support in RIOT.
While the reader is needed for test data, the writer is not used.
It would only be used by applications depending on jena-core without the rest of apache-jena-libs (specifically, jena-arq for RIOT). If RIOT is present, then the writer will have been rewired to be the correct one.
The overall result is that jena-core has RDF/XML, an N-Triples reader and a basicTurtle parser.
jena-core is not intended for standalone use. We have has apache-jena-libs fora long time now.