I'll add some perspective from an alternate reading of the primacy rules.
What is the official stance?
That depends on how you read the text included in each errata. While there are several versions of this errata blurb, each states the "primacy" rule,
When you find a disagreement between two D&D® rules
sources, unless an official errata file says otherwise, the
primary source is correct.
But, you must continue reading, as while there are different versions, nearly every version gives an example, like
One example of a
primary/secondary source is text taking precedence over
a table entry
For example, the table for Armor Class Modifiers under Favorable and Unfavorable Conditions has a footnote that appears on Helpless and Pinned, but the footnote mentions penalties that don't apply to pinned.
Treat the defender’s Dexterity as 0 (-5 modifier). Rogues can sneak attack helpless or pinned defenders.
I suspect the intent was to tag the latter portion for both, but it reads as if the penalty applies to both. However, the associated text for the two conditions is clear that that penalty (Dexterity as 0) only applies to helpless. (Different penalties apply for pinned that the table does not mention. The penalties are close enough in practice it will make little difference, unless by a misreading you allow a pinned opponent to be subject to coup de grace.)
That is what the errata blurb is intended to clear up.
If you read the primacy rule as establishing a hierarchy where the earliest publication takes precedence, the problem arises when an intended change or expansion is a fundamentally different than a previous publication. The blurb is an acknowledgement that the publisher makes mistakes. But intentionally expanding or changing rules shouldn't be viewed as overruled or somehow prevented by previous publications.
This prevention interpretation of the primacy rule also means that sources such as FAQ, Rules of the Game articles or other materials from authors and designers are viewed as invalid in their entirety. I would argue any advice needs to be considered carefully, especially if contradictory, but not because of the primacy rule, rather, because by not being in the rule books, they might add confusion. But that's another topic.
While in some cases the RC adds new rules, most of the compendium is a compilation of rules from the core rulebooks and other expansion books. This makes it neither unique nor the first to modify rules intentionally.
For example, the rules for charging versus the rules for Balancing that expand on charging are from the Player's Handbook, or the expanded rules for hiding are sourced from the Complete Adventurer Expanded Skill Descriptions, pp 101-102. Without the Rules Compendium, you have to look to the rules in Magic Overview and Casting in Combat and Actions in Combat to determine that unarmed "armed attack" qualifies for attacks of opportunity or both read and heft around all your splat books to derive modifiers.
What's particularly provocative about the Rules Compendium for the prevention interpretation is that it states up front that it supersedes previous rules.
Years in the making, it gathers resources from a wide variety of supplements, rules errata, and rules clarifications to provide an authoritative guide for playing the D&D game. It updates and elucidates the rules, as well as expanding on them in ways that make it more fun and easier to play. When a preexisting core book or supplement differs with the rules herein, Rules Compendium is meant to take precedence.
This is in stark contrast to previous splat books that present the material as an accessory to the game that DM's may use. The RC presents not as an accessory, but as the definitive source, implying the rules additions are no longer optional. With accessory rules inline with the rules sourced from the PHB or DMG, it is near impossible to selectively use accessory rules without being intimately familiar with all the sources. This is a problem from any viewpoint on the primacy rules.
To address Hey I Can Chan's comments and post:
Charging through Hindrances:
This isn't necessarily a change or modification, rather suggesting using other pre-existing rules to get around not having a clear path.
Touch Spells and Threatening an Area
This is from the PHB, Magic Overview, Actions in Combat and Casting Spells in Combat. One says you are armed, one says you don't provoke and the other says armed "unarmed" threatens.
What It Means to Be Hidden
From Complete Adventurer Expanded Skill Uses.