I’ve never really thought of it like that, but I see what you mean.
Even if the objective is similar, I’d always thought they were two different approaches.
With ETTR, I’d always considered it primarily as an approach for dark scenes where you are deliberating over-exposing to try and capture as much light as possible, which you can then darken in post to get the “right” exposure while keeping noise to a minimum.
With exposing for the highlights, I’d considered it simply as an approach for bright scenes where you prevent clipping by exposing for the brightest part.
But on reflection, you are essentially just doing the same: exposing as much as possible just below the clipping threshold. Thx.