It seems intentionally worded to provide plausible deniability. It's one thing to say "don't assume we will always dominate search" and another to say "don't assume we currently dominate search".
The latter "assumption" is an outright falsehood (in the context of the overall search market).
Bizzarely heurestic delusions and self-deception can be useful for taking to heart something which isn't literally true.
Take "the gun is always loaded" rule. Now nobody thinks it means that if they pull the trigger repeatedly on a gun with no ammo they will cause an empty gun to occasionally fire a shot or two after cycling through it many time. It means always treating it as such to avoid any slipups from when it is "not loaded" when it has one in the chamber even after removing the magazine, one loaded from the magazine after clearing the chamber before removing the magazine, etc. The costs of treating an empty gun you don't intend to use as loaded are nothing compared to a mishandling so it is perfectly rational.
In this case it could be argued a similiar thing for the company. Assuming they currently dominate search when there are up and coming ones, subsections which don't technically fit the same category and can take them by surprise (voice assistants like Alexa taking that role is quite optimistic but plauisble enough that consumers could wind up preferring it), or similiar?
It would be "monopolistic" in that it helps keep them on top but "anybody" can self-delude to drive their actions so it isn't anticompetitive any more than say being the only jeweler in the country with fifty years of experience is.
Fair points, and I agree this mindset is probably a good one to have.
But the document in question is not a training document for the purpose of curtailing employee complacency and remaining vigilant in the face of competition. It's a document specifically discussing ways to avoid anti-trust regulators. That's an important distinction.
If employees already believed it because of their internal training, there'd be no need to "remind" them what not to say.
The latter "assumption" is an outright falsehood (in the context of the overall search market).