OK, so the mistake isn't 6-8 encounters per day (which presumably takes into account the diverse range of resources available to different PC classes), but how difficult those encounters should be? Maybe. Or maybe 'adjusted' shouldn't be in the titles of one of the charts. :shrug: Neither way really inspires a lot of confidence.
Or maybe we're completely misinterpreting the whole thing, and neither classes nor encounters are meant to 'balance' at all. ;P
If you look at the math of 5E it seems clear that classes and encounters are "meant" to balance in combat--they tried really hard to build a structure which would deplete PC resources at a predictable rate. For example, the XP multipliers are basically an implementation of Lanchester's Square Law (not that they necessarily got the idea from military theory, since it's a fairly obvious analytical technique) which predict total loss to a force over the course of a combat: losses scale with the square of the size of the enemy force, up to a point. Or look at the shape of the DMG CR table, and the guidelines they give for CR: they practically come right out and say that DPR and HP are the only factors they care about. (Mobility is worth zero, according to the DMG.)
Just because the 5E designers apparently tried to create a balanced system doesn't mean they succeeded, of course. In fact the whole system is completely broken from a balance standpoint, in that there are tons of brokenly good strategies and tactics that let you vastly exceed baseline expectations. (Another Enworld thread recently brought up the fact that a 1st level wizard can potentially solo kill the Tarrasque through judicious use of Longstrider, his starting gold, and Acid Splash. Back in AD&D you needed to be at
least 7th level to kill the Tarrasque, and 9th was better.)
I'm 100% fine with that state of affairs. 5E is interesting to me in the same way as
Master of Magic: not only is it possible to utterly break the game difficulty, but there isn't a single dominant strategy for doing so. You can break the game in a dozen different ways, and each of them gives a different experience. Therefore it is fun, or at least it has been so far.
(As an aside, if you've never played Master of Magic, you must try at least four things: evocation Channeller with Flame Strike to nuke enemy armies from afar; eleven-book Black Magic wizard to create armies of self-replicating Ghouls and Lycanthropes that are immune to normal armies; Rune Magic + Artificer to create invincible heroes who solo entire armies; and fleets of flying invisible (spell-locked) warships bombarding enemies from the air with catapult stones. All of these tactics are game-breakingly good, which leads to a metagame of breaking the game in new and different ways.)
TL;DR they tried to balance it, but failed, and if they'd succeeded 5E would be boring (IMO).