You also don't understand that Keldeo has zero use outside of Calm Mind or Choice itemsMaybe if you had read my moveset a bit more carefully, you will see that Scald was chosen for Defensive purposes. As I said earlier, and clearly need to repeat, Scald was only listed as an option if Substitute is used on the Calm Mind set, and so that the Burn can make Keldeo sorta wall Physical Attacks alongside Special Attacks.
As for Surf vs Hydro Pump, what good is all that extra power going to do if it's PP drained by something with Pressure? Take any Hydro Pump equipped Keldeo against 1 well trained, perfectly EV'd Spiritomb with it's pick of Protect, Calm Mind (combined with Drought Ninetales for maximum effect), Brightpowder, and/or Rest/Sleep Talk and that Hydro Pump is rendered useless. (So is Stone Edge and Fire Blast but Keldeo can't learn either of those.) If Hydro Pump had better Accuracy, I'd be a bit more open to using it, but anything less than 90% Accuracy is never worth the aggravation. I will always go with slightly less power with perfect Accuracy over unreliable power moves that miss right when you need them, and always run out of PP in the middle of a key fight.
Tyranitar was not mentioned as a partner for Weather benefits, if you had read that carefully. I never said Tyranitar was an ideal Weather partner, but it's a solid Defensive partner. So there's absolutely no room for error what I mean, I'll list how & why I chose Tyranitar as a potential teammate, going in detail over the Defensive benefits it provides Keldeo.
Electric: Keldeo's alphabetical first weakness. The majority of them are weak to Earthquake and most Tyranitar carry it as a matter of fact.
Grass: This one is a bit trickier due to being a common weakness to both Keldeo and Tyranitar, but unlike the horse, Tyranitar often carry something that's Super Effective against Grass types, be it Ice Beam or Flamethrower. Keldeo, on the other hand, must rely on Hidden Power (or Icy Wind) if it wants an Attack of that type.
Flying: Due to the prominence of Bug and Fighting type Pokemon, Flying type Attacks are going to enjoy a surge of usage, and next gen is probably going to have many good Flying types to balance out the sheer number of good Bug and Fighting type Pokemon, Keldeo among the latter, this generation brought us. Tyranitar has little to no trouble dealing with Flying types on it's own. Either a STAB Stone Edge/Rock Slide or the aforementioned Ice Beam from Tyranitar and most Flying types are insta-KO'd, leaving them unable to stop Keldeo from sweeping.
Psychic: If there's one thing Tyranitar does better than change Weather, it's destroy Psychic types. Crunch, Pursuit, Dark Pulse, STAB on all the above, strong Offensive stats, and immune to their STAB attacks, few Pokemon can handle Psychics like Tyranitar can. If a Psychic type is sent out to handle Keldeo, the horse should be hiding behind a Tyranitar.
As you can see, Tyranitar on it's own is a match for/more than for most of what can threaten Keldeo. Therefore, why I mentioned it as a potential partner. (Of course, this may be a time where it's less useful Ability would be preferred, since anyone can see that Sand is not ideal with Keldeo.)