There’s no errata for the barbed devil (Monster Manual 51), and the issue the question raises is unchanged by the premium edition of the Monster Manual (2012).
According the stat block the barbed devil does, indeed, possess but does not meet the prerequisites for the feat Improved Grapple (PH 95-6).1 Further, this discrepancy also goes unmentioned in, for example, John Cooper’s otherwise excellent Cooper’s Compendium of Corrected Creatures.
The easiest thing for the DM to do is say that barbed devils just can’t use their Improved Grapple feat, the DM ruling that barbed devils simply somehow came to possess a useless feat. Almost as easy is the DM making a house rule saying that the Improved Grapple feat of the barbed devil is now a bonus feat (so that even though barbed devils “do not have the prerequisites for (the) feat… the creature(s) can still use the feat” (7)) and giving the barbed devil another, reasonable feat that’s unlikely to change its challenge rating.
There is a another tack that can be taken, but it’s more difficult and comes with some baggage. While the Monster Manual on Feats really does straight-up say, “A creature cannot have a feat that is not a bonus feat unless it has the feat’s prerequisites” (7), Oriental Adventures on Feats amends and softens this absolutist stance a little, saying
Certain monster special abilities count as feats for purposes of meeting prerequisites for other feats. A monster with any natural attack form is considered to have the Improved Unarmed Strike feat. A monster with the improved grab special ability is considered to have the Improved Grapple feat. (143)
(Link added.) A strict reading simply ignores the rule above: Oriental Adventures is not a primary source, and, as this rule contradicts the Monster Manual, it is safely and easily discarded. Alternatively, a reader going to Gamer Court could make the case that the rule above only applies to Oriental Adventures creatures.2
But a looser reading can accommodate instead of dismiss the Oriental Adventures rule, viewing it as expanding and clarifying the Monster Manual‘s rules instead of contradicting them. In fact, while attempting to make such an accommodation, the barbed devil may even be used as evidence that the Oriental Adventures rule was a thing all along!3 Such a reading does solve the barbed devil problem, but it also makes it so, for example, any creature with a natural attack or the special ability improved grab gets a secret and otherwise-unstated-elsewhere natural ability that allows it to meet prerequisites and requirements it otherwise couldn’t.4
Keep in mind that while it’s unusual for the game to secrete new or updated rules in unusual, often unread places in ancillary texts, this isn’t wholly unprecedented (cf. the accepted answer to this question).
What this DM would do
This DM imagines the designers did, in fact, want the barbed devil to be able to use the feat Improved Grapple as the feat was assigned to the barbed devil. I assume that a deliberate design choice to give a monster a feat that it can’t use would probably be mentioned in the monster’s text. I also suspect the barbed devil’s challenge rating includes the barbed devil using the feat Improved Grapple rather than its challenge rating including the creature possessing a useless feat! For this reason, I’d avoid the Oriental Adventures rules, make a house rule that adds the B to the barbed devil’s Improved Grapple feat, and add to the barbed devil the feat Stealthy (PH 101), a feat that shouldn’t have an significant impact on the barbed devil’s challenge rating.
1 Now that I’ve looked for it, the feat Improved Unarmed Strike is also absent from the Monster Manual‘s other creature with the feat Improved Grapple: the bebilith (42)).
2 Despite context implying otherwise. That is, the paragraph after the rule above begins When a monster described here…, a turn phrase omitted from the Oriental Adventures rule quoted above, therefore a strong (but not airtight) case for the rule above seeming to be intended as a general rule.
3Oriental Adventures is, in fact, the original source for the feat Improved Grapple (63), the feat having been later reprinted in Deities and Demigods (51) and Dragon #290 (91), but in these cases lacking the burdensome Dexterity 13 prerequisite that was added by Unapproachable East (May 2003). Sadly, it was this last version that was adopted by the 3.5 revision of the Player’s Handbook (July 2003).
4 Note that this is, in most cases, really not a big deal. However, the DM risks opening a Pandora’s box of subsequent judgment calls if the DM extends the Oriental Adventures rule to cover feats beyond Improved Grapple and Improved Unarmed strike!