To be fair, that was Thrall's Horde that drove off the Centaur and that does not exist with Sylvanas' current reign, not even the same Horde, nowhere close. She wasn't even there in that time period to help his people anyway so he owes her jack shit. The only fool here is you for not remembering the history of the Horde very well.
Proper leader? Do me a favour and either look up the Shaman class hall scenario with Magatha or do it yourself, Magatha is very, very power hungry and wanted to keep and use a very dangerous and powerful weapon that the Earthen Ring forbade. She does not embody the Tauren's values and culture at all. Do you even know anything about the Horde's lore or are you just another Forsaken shill? The Tauren of Thunderbluff would sooner die fighting her than let her lead them.