Blitzcrank grab hitbox

It's pretty wide and you were walking towards it. Front edge of your hitbox found the back edge of the pull it looks like. Frankly, not surprised it hit you at all from that replay, though I've been around the game far too long and generally have a decent idea of when it will/won't land, visually.

You should hop into the practice tool with Blitz. Turn off cooldowns, plop a teemo dummy down, and play around with it to see what will/won't hit.