I'll just throw in one more thing. Playing 'in tune' is not always about perfect intonation, but there is a big difference between deliberate deviation for the purposes of musical expression or interest and just being 'pitchy'. Sliding around searching for pitch is very different to a well executed vibrato. Making deliberate adjustments to 'sweeten' notes over time, in particular major thirds, is quite different to covering up errors with 'grace notes'. A bunch of slides is quite different to a true melisma such as a vocalist or sitar player might do.
All of these things are part of the language of fretless bass, with or without lines.
+100!
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