The facts behind such a statement are as such:

PLD have pretty bad snap aggro. Meaning they can't pull very well at all. (Often mitigated by having the WAR or DRK pull and then swap if for some reason the PLD is MT)

Outside of that, they're fairly equal to both other Tanks in terms of MT role. WAR edges out because Holmgang lets them one button ignore TB's more frequently than any other Tank. But otherwise the 3 Tanks can all mitigate damage adequately.

On the flip side, PLD has plenty of tools that allow them to be effective while in the OT position. They can Cover people, they can use Intervention on people (Also, enhance it with Rampart and/or Sentinel), they can use Passage of Arms to mitigate a bunch of damage in an AoE and they can shield people with Divine Veil. Heck, in a pinch they can even use Clemency and heal people.

Meanwhile... WAR can... Shield people with SiO and... Slap the boss... DRK can... Shield people with TBN and... Slap the boss...

So, essentially, while PLD's are functional as MT's, given that their only weakness can be covered by the other tank... Both other Tanks are absolutely trash in comparison to PLD in the OT position. Since PLD has far, far more utility that they can bring to aid the party - Even if it's not necessary to rely on these tools, the fact that they exist and can be useful at no loss of actually bringing the PLD makes them the go-to OT.