No, I am 100% sure it is flash. I think it was on old site. Additional detail: it had S.Noni tileset and music and had same simple line drawing.
//EDIT: And I joined after switch to Flash so I missed java-based games and yet played this grappling hook via flash (I had problems with java in browser at the time I played it, so it certainly wasn't applet)
//edit #2: Yep, it was on old site and forge before wipe as JnR Addons.