And being dishonest does?
There's also a lowest common denominator effect, where the other guy's marketing makes false claim X, and if your marketing doesn't, you are at a big disadvantage. Most of the time competitors aren't dumb enough to make a claim that is so blatantly false as to be legally actionable; as long as they steer clear of that high bar, they will make a lot of gains by making a misleading claim (note: they may make a lot of gains even if they can be held legally accountable, as that is a slow and expensive process, and the expected cost/penalty may fall far short of the expected profit).
1) Their product hasn't launched yet.
2) They desperately regret using Mongo and are trying to get rid of it.
Everything is a mind game. People have identified their use of things like Mongo as being on the forefront of a developing technology, it makes them feel important and interesting. Try taking that meaning away from them and see how it goes for you. The practicalities of actually using the thing hardly matter.
And is the guy who initially advocated for Mongo going to show up with his tail between his legs and admit he made a mistake? Nope, even if he wants to, that would be a big hit for him career-wise and after our mid-20s most of us have been disabused of our egalitarian notions and know better than to do that.
RethinkDB is the "good engineering" counter to MongoDB. Didn't oversell, worked hard to build a world-class product that targeted the same general product class. Compare for yourself and see what you get by proceeding with an engineering emphasis.
Marketing is mandatory, and developers are naive if they believe they or their field is immune.
The recent discussion between Sam Harris and Scott Adams might be interesting to you. Take into account that moral and good are synonymous with truth, to Sam Harris (strict rationalist) and Scott Adams is a consequentialist/utilitarian.