The reason rulings not rules approach works with 0D&D is that there simply are not rules for the situations that might come up in play, so the job of assessing them and deciding the outcome is put into one person's hands.
Games with complete rules covering what the game is about generally don't need the approach. Given that 5E has a pretty robust rules framework, I'd say that this approach is nothing more than a common refrain in games from the 90s that it's the GM's job to make the game work by ignoring the rules rather than designing rules to support a desired type of play.
I wonder if the Old School guys are going to get sucked in by this propaganda. Will they start cheering "Mearls finally gets it! It's about rulings not rules!"? Not realizing that the fundamental underpinnings of the game that support rulings vs rules is not a robust & universal resolution system like 5e has?
I'm beginning to think that the modularity talked about so far will simply be DMs being told to use whatever they want rather than well developed rules modules meant to produce a certain style of play. I'm guessing that these ideas will be culled from playtest suggestions rather than being designed from the ground up to produce certain results.