It can be considered that way, and I called it as much myself at the time, but that's mostly just because of how broad a set of parameters would have been "PLD" at the time. Namely, before 2.1, "PLD" included any sustain (means of surviving, be that healing, barriers, %DR, %recovery, etc.) that scaled with enemy attacks instead of solely your own stats.And while they certainly didn't need to copy-pasta defensive CDs or thereby line Warrior's defensive structures up with Paladin's, they absolutely did need to make WAR's sustain scale less with its own stats and more with incoming damage. Else, had it simply been tuned to compete at mid-progression or late-tier, it would consistently be inferior until and superior thereafter, even allowing their party to forgo a healer in dungeons and certain boss fights after that point. Barring gimmick solutions like much stronger raid buffs for avoiding repeat jobs, you'd ideally just double PLD until the balance point, perhaps briefly use both, and then double WAR unless there was particularly good Hallowed Ground cheese possible in the given instance. You'd have Early Tank and Late Tank more so than simultaneously competitive Tank Style A and Tank Style B.Yes, technically, MRD had one such ability (Foresight), which Paladin could then also take, but it was worth only around 3% mitigation at the time WAR was changed and, again, wasn't unique to Warrior, unlike Sentinel, Bulwark, Hallowed Ground, and the ability to block enemy attacks. Warrior's only mitigation tools that anyone could at all notice were Rampart and Flash, both Gladiator skills.
You don't need each fight to have a preferred tank. Heck, doing so means you increasingly end up with one tank choice at a time, just with rotating face.
You do, however, need them to do those things in different ways. A GNB being able to kite mob damage, pounding into a displaced focus target while avoiding the rest before bursting CD-based AoE against all to survive that through suppression from damage dealt... is still X sustain + X damage, possibly even following very similar scaling factors and profile to other tanks', yet would feel very different from a Paladin alternating turtling and reprisal or a Warrior weaving between Defiance and Deliverance abilities to survive while dishing out as much as they take, etc.