Your guess is to my knowledge the answer to your question. Despite the GUI, Octave has little dependence on qt. All mathematical or input/output libraries are independent of qt.
To verify this, I had a look at the HAVE_QT build macros, which are guarding code depending on qt on the Octave 6 stable branch (some .hg and cache files are stripped from the output):
I asked that not only for curiosity, but also considering situations where qt tools/libs are not available (system with low resources, old qt version, qt tools not in the repos, etc…).
Without Qt won’t be able to print with the -svgconvert option that brings a few improvements for vector formats (support non ascii characters, transparency).