I’ve had other bass players tell me that I get a fretless sound out of a fretted bass, but, honestly, I wasn’t always sure what they meant. The last time I remember this happening on a gig I was playing a Ken Smith BSR5M through a Genzler MG350 into a Genzler BA12-3. As far as playing style, I came up playing upright bass, and I use a fairly strong right hand pluck to really get the string moving. I adjust my plucking position as needed but often end up over the bridge pickup. I also pluck with as much meat of my finger as I can, not just the tips.
Regarding left hand, I try to place my fingers right behind the fret, not at some vague point in between frets. I also try to hold notes for their full duration, letting them ring. I deliberately avoid fret clank, simply because that’s not the sound I’m going for.
I’m not sure if any of this has bearing on the original question, but that’s what I do when achieving what I’m told is a fretless sound. I do play fretless, and to me, it’s a very different sound than what I get on fretted.