So far the OP is stating a common situation WRT the cable's initial "tension" (actually it's length, the tension is up to the der's return spring). What I don't see in the vid is trying to change the adjuster's position within the shifter, "tightening" the cable. Not sure why the OP didn't try to unthread the adjuster more then they did, only a brief touching of their fingers to the adjuster but no obvious effort to unscrew the adjuster.

I do try to set the cable clamping so that there's less cable slop to begin with. This way the adjuster only has to be unthreaded a small amount for good "tension" and more adjusting capacity remains for future tuning. Andy