the player's original point is that it gives kalista a large presence in team fights compared to other ADCs
the only ADCs with as reliable damage to multiple targets are: AP Varus, Jinx, Sivir and Twitch.
Kalista's rend is non-targetted, applies crowd control, gains an instant reset upon any kill with it, stacks infinitely and does not have a limitation other than autoattacking.
AP varus loses a large percent of his dueling power in exchange for large %hp ratios
Sivir has to be within ridiculously close range for her auto attacks and only does damage, reduced damage at that to her multiple targets
Jinx does more monstrous amounts of damage and has hard crowd control, but her power spike can't happen at any point in the game
Twitch is sheerly a late game champion, kalista's ability was almost literally copied from his, his ability has a cap, does minor damage(though lasts a long time) and he has to waste his ultimate to have a team fight presence, a large issue if he is taken out of the fight early
compare that to a champion that is effective at any point in the game, has a moving ward to prevent ganking, has perfect synergy with a tank support, an ability with infinite stacking damage and unlimited mobility provided there is a target. I don't know if her Q allows her to dash as he points out, but if it does, that means that she has an untargetted dash as well, effectively providing limitless escape opportunities while dealing damage
and as a fun bonus, kalista can save her support or give her support and/or herself an instant, stunning engage