Mearls never said that you could have a table of gamers play the game just like 4e, and have a player come in and play their PC just like 1e with no problems.
I think he said something more like, /one/ player of each ed, rather than an odd-man-out scenario, but my recall isn't perfect.
They took down the old L&Ls, or I'd link it for you.
In order to run 5e like 1e, I imagine there are just as many major changes needed mechanically as if you wanted to play it like 4e. I don't play 4e, so I can't comment on the changes, needed, but I disagree you could run 5e like 1e with no formal adjustments, and that assertion makes me wonder if you remember how 1e was played.
It was played pretty differently at each table, as I recall!
When I say 'run 5e like 1e' I mean get the same feel, use an old module just converting on the fly, stuff like that. Not translate all the mechanics to make 5e /into/ 1e - which'd be silly: just play 1e, I still have the books, and they haven't fallen apart.
For example, going from ascending AC to THAC0 is a pretty radical change.
Simplicity itself. AC 10 is 10; 3 is 17, 0 is 20 etc. In theory -10 would break 5e like glass, but in practice no monster ever had that. If a monster is problematic to on-the-fly ruling, I'd just go to the trouble of looking up the current version.
So is removing all non magical healing completely.
You don't need to, it has no impact on the feel of play. Though, you can just use the 'gritty' short/long rest if you want to drag out the pacing.
Edit: Ok, that may need some reasoning, I realize it's quite a claim. Thing is, back in the day, you'd grind against a dungeon a bit, and then 'go back to town' to rest. Some DM's'd let you barricade yourself in a room or something and rest. You fought as much as you could, rested as much as you could /to recover spells/, sometimes in cycles, and moved on. The excruciatingly slow natural healing rates never came into it. You had your Cleric systematically cast Cure..Wounds or whatever until you were all healed up, maybe that meant 'resting' two days in a row, even, but that's as slow as it ever got. (I've seen 1e played /lots/ of ways, but /never/ the cleric-less, weeks of recuperation - barring 0 hps & a DM who enforced those rules - some people seem to think was the only way anyone ever played it.)
So, no HD and overnight healing don't break the 1e feel of 5e. They zip over something that was boring and usually moved over pretty quickly in 1e if you could manage it.
As is getting rid of death saves.
They serve the same purpose as negative hps. Down & dying. The feel isn't appreciably different.
And then adding in save or die effects.
Again, nothing to it: save mechanics are in place, the narration of failure is just different.
Then spell memorization/failure and spell interruption.
Memorization instead of slot casting is easy, old-school players trying 5e for the first time will do it instinctively, you have to train them not to.
Taken care of at char pregen - just don't toss any criminals who aren't Rogue(Thief) or the like into the pile for the players to choose from, for instance.
And class specific leveling.
Irrelevant w/in a session.
And the entire skill system (removing it pretty much except thieves and assassins). So yeah, in order to play like I play 1e, there are major adjustments that are needed.
1e didn't have a skill system, but every DM grafted in something or just used his judgement and narrated results when players did thing that, in 3e or 4e, would require a skill check. That's /exactly/ how 5e runs, by default. The players declares he'll do something, and the DM either describes how it works or fails, or calls for a check - the mechanics of the check are just consistent in the case of 5e (as they would be with a DM who consistently used on variant to resolve actions, like roll-under stat checks, for instance).
Saying you can play it like 1e based only on rulings vs rules and attitude isn't accurate because you can play 4e that way too
Maybe not 4e, but I did run Temple of the Frog under Essentials with pretty fair fidelity to the feel of the original.