That was flavor that didn't make it through the design team.
When they first appeared, on Legacy of Fire 4 (which was 3.5), they had the following description:
As this serpentine creature slithers forward, its body combining elements of snake, bird, and human, a visible shiver shudders through the fabric of reality. Its unblinking eyes reveal a patient and powerful intelligence, while from nowhere and everywhere at once resonates a susurrus of infectiously spreading whispers.
However, their most marked trait is the subtle telepathic whisper that constantly surrounds them, reaching out to touch the minds of those they encounter, capable of confusing the weak-minded and even temporarily altering their alignments.
Their sheet, however, is very similar to what we got on Bestiary 2. It was simply converted, skills changed, some feats changed, but nearly everything else is the same.
It seems that this ability should be somehow related to their warpwave ability, but all results on the roll (B2 it's a d20, in 3.5 it was a d10) are physical changes on the target. Other than that, it has a tongues effect permanently, but i fail to see how that is any more chaotic-inducing than trying to figure out what this flavor text is talking about.
Intrigued, i asked the author of most of planar-related material from paizo's team, Todd Stewart, and this was his answer:
I think that this was a case of the original flavor text being retained, but an ability reflective of that being replaced at some point along the way. I've always played them as surrounded by that whispering aura that may or may not be talking to anyone specifically near them (and in fact it might keep talking to you in dozens of different voices, all at different volumes, all in different languages, all from different apparent points of origin including inside your own head).
I'd have to go dig up my original draft of the imentesh, but I do know that my original protean trio ended up being tweaked and reworked considerably by several others before we ended up with the final stats in Bestiary 2. I'm considerably happy with where they ended up, let me be very clear about that.
Without looking, I strongly suspect that the warpwave ability (which is amazing but takes up a lot of space) ended up replacing a few other things, including a bevy of SLAs and probably an aura that dovetailed on that fluff.
The imentesh are also one of my favorite monsters to use just in non-combat encounters with PCs. I had one of them show up in the Maelstrom borderlands and just fall in with them, badgering them about where they were going (while actively bending the landscape around them to keep them walking in circles), and then keep showing back up at odd points and insist that it wasn't them, but another imentesh altogether who'd been making an ass of themselves to the PCs.
The original flavor he mentions can be seen on The Great Beyond setting book, which was written for 3.5, and describes the planes for their campaign setting:
The imentesh proteans seek the same, but through subtlety and persuasion, like wandering prophets of chaos infecting minds, philosophies, and even religions.
In Legacy of Fire, we see that Imentesh are not that chaotic, the creature there tries to reasonably discuss with the PC's (i will try to not spoil anything in the adventure for readers):
As she moves to greet the PCs, trusting her shape to keep them from attacking, she watches and waits to see what the PCs do. Lahapraset’s initial goal is to determine if the PCs’ curiosity and temper are such that they would be capable of reasonable discussion, or if they are merely mindless slayers and killers. If the PCs attack, she simply regards them with an expression of mixed disappointment and anger before resuming its true form as a free action.
There are three Imentesh in Pathfinder Scenario #04-20 "Worlds of the Ancients, with more information on how they behave when in a group or small community. Their abilities are exactly what we got on Bestiary 2, though.
And finally, Ultimate Magic also supports Todd's flavor ideas for them:
Imentesh (SR 21): These heralds of chaos often heed the call of binders who wish to utilize the power of proteans. In conversation they are often talkative, seemingly helpful, and polite, but they are always plotting their escape in order to unleash as much chaos as possible.
The answer from James Jacobs, paizo's creative direction was similar, but also adds more information:
It was flavor that we had to cut because it was both too much to fit on a page and it was a case of flavor text not being properly supported by rules, which is a problem we had now and then a decade or so ago when we were using the 3.5 rules and still finding our feet, I guess. We probably should have toned down even further the implications that their telepathy has weird side effects before printing them in Bestiary 2.
So, aparently, they didnt have a good way to reproduce this effect at that time, and decided to change the creature's ability to do something else, which explains the warpwave ability, that had almost no mentions back in Legacy of Fire.