Generally, the later rule takes precedence over the earlier rule, which is why I've heard that
O Elbereth! Gilthoniel! doesn't work (the rule against canceling the Ring-bearer's skirmish postdates OEG). Also, the specific rule takes precedence over the general rule (which is an argument for making OEG work).
Thus cards can override the rules if they have an action that would be invalid otherwise. For example,
Sent Back lets you play a unique companion when there's a copy of him in your dead pile, in direct contradiction to the rule that says you can't. For that matter, even cards like
Gollum, PD override the rules inasmuch as its text is apparently active when it's in your discard pile and not in play.
The only clear exception I've heard to the principle that cards override the rules is OEG (presumably because the new rule came after OEG). Some might argue that another exception is playing events out of phase with
Pass of Caradhras or
Bilbo, BoTB, although this is better explained by saying that because those actions do things other than play out-of-phase events, they aren't clearly intended to override the rules (which is why, in contrast,
Diversion can be played out of phase).
I would argue that
WitD falls into the same category as
Sent Back and
Diversion and overrides the general rule saying followers can't bear cards. You should note the errata, of course.