Precisely, level 30 is the key. The Secret Cow is in the Barrens, but level 30 on heroic also.
Well, the ‘secret cow level’ was a joke back in the days of Diablo, a rumour that was made true in D2. This whole affair is a reference to that franchise, as is the design of the level (all the regular enemies there are ‘hell bovines’… wielding halberds) and the boss (the Cow King in particular is a ‘lightning enchanted’ super-unique monster there, i.e. emitting lightning bolts each time he’s struck; some parts re Tyrael and the portal to Whimsyshire or whatever refer to D3, which I haven’t played either). While, as I understand, Tyrael and the whole affair in Mercenaries are a promotion of some other new game that I don’t care about either, it is a funny tribute to those old days (I would argue that D2 belonged to that era of ‘good old’ Blizzard in terms of game quality) as well.
It is my assumption that the reward pool, apart from Tyrael, refers also to the Paladin class in D2.
As a sidenote:
But why? I understand that it might be easier to do some tasks with him when that swordfish is not maxed, but there was a time when Cookie was arguably the most powerful mercenary in PvP, included in nearly every competitive team (‘La-la, barloopah-eh!’ 
), so having him maxed was beneficial, including the fishes, which were so powerful with the Cairne-Diablo combo, and, in my opinion, it far outweighed the minor inconveniences that it caused sometimes when doing tasks. Nowadays his relative power level doesn’t quite keep up with what it used to be, but he can still be playable.
Digressing even further, there are some pretentious… characters out there calling themselves ‘content-makers’, who sometimes make… ambitious 
claims about ‘the easiest way’ to do something (like they tried them all), for example, or how you should ‘never’ max certain mercs (Cookie or the old Garrosh, for example, which both used to be arguably much better off maxed for PvP)… My suggestion would be to take it all with a pinch of salt (especially given that these characters often give misleading advice, having no clue about certain aspects of the game, while presenting it all as nearly the ultimate truth) and use one’s own common sense and judgement instead.