You’re exactly right, just been reviewing the Levition wiring diagram and it indicates their Add-on/Remote with indicator requires 3 wires. Unlike GE remotes w/o indicator which require only 2 wires. Leviton remotes with indicators apparently need Hot and Neutral lines to power the ~5 milliamps to the indicator LED; plus the “traveler” wire to send the control signal. I was naively hoping the ~5ma could be carried on the traveler wire as 60hz while control I’m guessing is kilohertz.
But you’re also right, this is a retrofit of mechanical 3-way switch…perhaps not the most traditional arrgt, Line-Switch-Load Switch, but on paper, I’ve convinced myself can also power the remote LED. It looks exactly like the diagram for GE 3-way switches from jhamstead https://community.smartthings.com/t/faq-ge-3-way-wiring/74962?source_topic_id=54638 except the previously unused black wire in the 3-wire Romex now goes to the Levition Remote black terminal.
Thanks to all for your responses!