I do not agree, as one who plays both a lot in the support role, they both fulfill different roles in lane and in teamfights. Lets start with laning phase. Blitzcrank lands a pull lvl 1, okay thats a kill. Leona waits in her brush lvl 1, random AD Carry comes, she Q's and her AD carry can pick up the kill. Who gets the adv. Lvl 1? Blitzcrank. Okay lets go to level 2. When their bot lane realizes that a blitzcrank is dangerous, they stay behind minions. Blitzcrank cannot land a hook and now is going to have to be content with blinding hooking the enemy brush for the support. They realize Leona is dangerous as well, but what can Leona do? E through the minions <---- This is HUGE for netting early kills. E Q lvl 2 combo is amazing if your AD Carry is someone like Graves/Ashe who can AA and use an ability the proc your passive twice. Easy kills. Leona gets it. Jump forward, level 6. Blitzcrank has his full combo, but AGAIN, he can't get a pull through minions, he has to resort to leeching exp or trying to walk up and take some hefty damage. Leona level 6, she is a tank, and has decent HP to her recently bought Heart of Gold. She combo's the enemy AD, and using her full kit along with her AD Carry net another kill or two. Leona wins. Lets go to ganks. Level 4 ganks or so. The allied ____ wants to gank for your lane, you are Blitzcrank, you flash Q and grab the enemy AD Carry, your jungler and AD carry follow up and grab a kill, results: burned flash, got kill. On Leona, without using summoners, u force an engage by bringing the fight to them, immediate W E Q combo buys enough time for your jungler to come in and get the kill. Fairly even, but Blitzcrank suffers from not having consistent opportunities for grabs. Okay then, now we go to teamfights. Both teams are grouping mid, you are Blitzcrank, as your team pushes the wave, you see an opportunity and take it, grabbing their AP Carry and netting a kill, this turns a 4v5 fight on, your team winning it eventually, on the side flip, you grab for a carry and instead grab their Jungle Amumu, he immediately ults and their teams mops the floor with you, suffering almost no losses. Leona teamfight, you are pushing their tower, but your Ryze gets caught badly, bringing him down to VERY low health, but you know he has spell vamp and AoE, so you go in anyway, you use W and E, hitting their entire team, while Q'ing the AD Carry, while they are still fighting you ult on top of their AD/AP Carries, stunning them and slowing their team. Your Ryze and team take advantage of your passive and the CC'd enemies and smack them over the head. On the flip side, you go in as Leona for no reason really. You give them a free kill, maybe, since you are a tank, you force a few summoners. Your team is forced to hold the enemy off your tower until you respawn and come back. Leona can win this.
Keep in mind different situations yield different results, and both of these characters are great in different situations. That being said, Blitzcrank is a versatile but limited champion, not being able to rely 100% on his abilities to hit the target he wants (Q cannot go through minions/players, passive on R is RANDOMIZED) But Leona is a CC tank jack of all trades in terms of versatility and adjusting to situations on the spot.
TL;DR: Both characters are good at certain things, but Leona's utility kit outmatches Blitzcrank's because of its less limited abilities.