This sounds like a hierarchy of category types, all components of which are character abilities. I mean, the article says, "A priority ranking for D&D is a list of character option categories, such as classes and proficiencies."
Mike isn't talking about the power of each ability you gain being better/stronger the higher you are up on the list. Rather, Mike is talking about the importance of the focuses of your character being stronger, or easier to gain or use higher up on the list.
So like he says... take invisibility/stealth. For them... it was more important that the stealthiest a character could be (in the easiest form) should come from class (as it is the highest priority.) They wanted the Rogue class to be the stealthiest, rather than the Invisibility spell giving you the best overall stealth. In this case, Class has a higher priority than Spell. It's their way of solving the problem that many players had in previous editions, which was spellcasters not only stepping on other character's toes... but also outright stomping on them.
I'm pretty sure something like Opening Locks would fall into this same situation-- when it came time to figure out the power of all the ways of opening a locked door, I'm pretty sure they wanted to design it such that the Class (in this case, again, the Rogue) had the strongest/easiest/most successful time of it, than say the Knock spell. Which makes all the sense in the world. They don't feel that a standard Wizard with a Knock spell should be hands-down better at opening locks than a standard Rogue.
Now, as he said... if a PC wanted to spend several different resources to reach a certainly level of power with some ability, then yeah it was okay for them to surpass the run-of-the-mill class who didn't focus on it. Like Mike said... a non-Ranger who took a certain background, feats and spells might be able to be a better tracker than your standard Ranger... but that was okay because of the amount of resources spent. But by no means would they want a single background Trait, or single Spell, or single Feat make you better at tracking that the one ability the Ranger class gives you to do it.
By the same token (although I don't recall any abilities yet in the game to give these to you)... they probably don't want any background, spell or feat to give you a better chance at what you can do with Stonecunning than that ability you get for being a dwarf. Selecting that race should have some importance... but that doesn't happen when you can be better at what dwarves do just by selecting a single ability elsewhere.
If the PC decides to select several different abilities that make them better overall at "stonecunning" like actions that your prototypical dwarf? That's fine. But at the same time... that self-same dwarf could also probably select one or two of those abilities himself and be the strongest/best character overall.
And that makes total sense to me.