The only tank that actually brings anything of value under the specific condition that they are not holding the boss is PLD, by virtue of Intervention and Cover, which are functionally nearly useless while tanking the boss unless somebody is screwing up badly.
What you describe is a situation that implies an awareness of what is optimal which means that it would not apply at lower percentiles. Regardless of the reason or situation, there is not a percentile or skill level measured on FFLogs that speaks to DRK having particularly higher tank stance DPS.
At high percentiles, a 100% DRK MT would likely be Gritless, rendering this discussion moot. At really high percentiles, there's no such thing as a 100% MT anyway, regardless of job.
At low percentiles, your argument implies that groups are aware of the fact that DRK is a poor OT. But it isn't really, its not any better of an OT than a MT, neither is WAR, tbh. As I said, the only tank that actually brings something to the raid while being aggro free that they would otherwise not is PLD. There isn't much reason to assume that poorly performing groups wouldn't just stick to the tired PLD=MT mentality, or that the WAR MT watched too many Xeno vids and tried to Deliverance it with no CDs resulting in SICK-DEE----*smack*---[dead]. I'd argue these former two scenarios are far more likely than such a group being savvy enough to realize raid dps would probably be higher with a turtling DRK. Bad players are also probably just as likely to believe TBN is valuable raid utility and relegate their DRK to OT-TBN spam. There's frankly no way of knowing what goes through these players minds. Its pure speculation. What isn't is the data.
Anyway, DRK's DPS being the lowest of the tanks is not really something that is disputable, differences in player skill and gear notwithstanding. Even its SSS dummy has less HP than PLD's, who also has no slashing debuff on a dummy, and also misses out on Shield Swipe procs on a dummy. This by itself of course means little, but in conjunction with flogs data... its not even really up for discussion. The giant dark magic sword does less damage than the 1-handed toothpick and the oversized BTN main hand.
¯\_(ツ)_/¯