Well, DK's do focus on using their melee-range weapons to deal most of whatever damage they do, so they are classified as melee. It's not so much semantics as it is deductive reasoning/logic. DKs use their melee weapons to deal most of the damage, be it Shadow or Frost, so they must be in melee range. Purely physical classes like Warrior also has to use melee range weapons to deal most/all damage, which we classify as melee. Both rely primarily on their physical close-ranged combat skills and damage to do their roles. Thus, DK is melee by default and not caster.
Since WoW is brought up, I guess the primary example one can give on the matter is Paladin and Druid there. Do you consider Holy (healing) spec to be a caster? If so, do you consider Protection or Retribution to be casters? They do use spells afterall. I'm going to assume "no". If we're going on the cast time/bars bit, then what does that make Resto Druids? Their primary means of healing is instant cast HoT spells, where it was pretty rare to see them actually use a cast bar throughout various expansion pack periods of the game before. They obvious had some spells with cast bars, and it would only be there for a couple, but it was hardly a heavy occurrence. I'd argue it's not much different than how often you'd see a cast bar on PLD here. I'm going to again assume something, in that you'd consider those Resto Druids as "casters", despite it commonly not casting spells involving cast bars.
Thus we get to my point on that matter, and what I honestly believe is a universal objective acceptance of what we consider a "caster". They are classes/jobs that are magic users where magic spells (in general) are their primary function, and thus we are able to separate physical combat oriented cases like XIV's PLD from it, despite it have magic spells available. This is akin to the differing of Protection and Retribution specs from Holy in WoW. Heavy cast bar requirements being present for most/all spell casts for the term to apply becomes very subjective, as is the case with my mentioning of Resto Druids.
And just to note, I left out "primarily magic users" at the beginning of my reply you quoted, so while we do have 2 magic using tanks, that's not how they function to primarily do their role. I did mention the "primary" bit at the end, in my defense. The tanks we have now focus on their physical combat skills to do their roles, rather than actual magic like you'd see from a BLM or SMN. Keep in mind that the term "caster" is never officially bound, and simply applied by common understanding. So you're not technically wrong to view it that way, but there is an underlying understanding about it. I laid out as much of an objective definition for the community as I could think of, but from there, the details are where it becomes subjective. I get where you're coming from on the idea of cast bars, since that's what magic users are often tied with (lore wise due to incantations and chants), but then we have to look at examples that break the mold, again referring to Resto Druid.
Edit: Just to add another clarification, I agree that the redundancy argument is pointless to say SAM can't be a tank. Over saturation of many elements that make up an MMORPG game are centered around redundant themes lol. Source: 13yrs of various MMORPG play, which include 12yrs of WoW and around 8 staggered yrs of XI.