I really, really don't want to get involved in this conversation, but it has to be pointed out.
The idea that
@pemerton couldn't run 5e "correctly"...like, right now...is...quite the claim. 5e is (basically) procedurally AD&D meets some repurposed/slimmed/discretized 4e tech + Bounded Accuracy + Adv/Disadv (present in tons of games including 4e) + Exhaustion (which is ripped from TB and discretized) + Inspiration/BIFTs (discretized version of Cortex+ or Fate tech) + Concentration (repurposed and extended 4e Stance tech) + Attunement + Social Interaction Loop (ripped from AW more or less).
So you take someone who is (a) an academic philosopher/lawyer whose professional life as about textual analysis + (b) someone who has run/played tons of AD&D and 4e + (c) someone who was around during the playtest so pretty much knows all of this stuff + (d) someone who clearly thinks intensively about TTRPGs. The idea that such a person would have any trouble at all resolving a session of 5e D&D is a pretty big stretch imo.
Simultaneously, though pemerton was very equipped to run Burning Wheel, I wouldn't have given him a chance to correctly run Torchbearer a few years ago. Not until he put in the work to understand not just the significant intricacies of the system (and how/where it diverges from BW) but how the entirety of each individual play loop (Town, Journey, Adventure, Camp) connects with each and integrates into a macro play loop to create the whole experience of play of the game. And then I still wouldn't have trusted him to deftly execute both Adventure building + the game's particular requirements of the GM on handling Fail Forward (how to manage the Twists or Condition + Success model and when to deploy Gear Twists vs the various other Twists etc).
As the saying goes, "there are levels to this" (going from one edition of a game to another or even going from one game within a family of games to another). Someone with little to no exposure trying to go from 4e to AD&D or vice versa? Not an easy thing. Someone trying to execute 3.x Encounter Budgets at level 11+ cold turkey? Good luck (I mean...even the most seasoned users have enormous trouble there)! Going from a background rich with AD&D and 4e + understanding how the 5e playtest went? Running 5e for such a person is not a tall order imo (and I would think that should be cast as a credit to the designers...that was, afterall, one of their biggest aims).