It may present a problem, but that's far from certain.
Being on a major genealogy website will certainly bring users to "MyHeritage Community". But I think the effect on G&FH.SE will depend on the size and nature of that user base. It may be that exposing more people to Q&A sites will actually "grow the market" for such things. In that case G&FH.SE can position itself as an alternative, perhaps for the more "discerning" genealogist.
Update Mar 2017
I've popped back to MyHeritage Community a couple of times recently to have a look. I haven't tried posting there. I wouldn't say the site is thriving, but there is a steady stream of questions. Eyeballing it, I'd say it averages 2 or 3 questions a day, but can reach double figures or be zero. So maybe a bit more than G&FH.SE, but not substantially so.
The site seems to have acquired a strong central/eastern European focus. There are lots of questions regarding translatiion to/from German, for example, and plenty from Poland, Russia, Scandinavia etc. Relatively few questions concern UK/USA/Canada.
I'd guess (very) roughly half of posts get an answer, and mostly that occurs within a few days of posting. That's probably because there is still (as far as I can tell) no search function, so the only "easy" way to find older posts is to click "Next" a lot. The questions do appear to be indexed by Google, so questions can be found that way, but from within the site older questions appear dead and buried. Learning from answered questions is not going to be a big "thing" at Community.
So MyHeritage Community has clearly found a useful niche, and is helping a modest number of people with their research. It is probably becoming a valuable tool for European researchers, in particular. But given MyHeritage's apparent huge user base, the number of active Community users is rather tiny. The UI/UX are awful - no search, no help, primitive filtering, one image per question, no posting guidelines - and I suspect it turns off far more potential users than it attracts. Because of this, it doesn't function effectively as a proper Q&A site, but it does work as a translation and "tell me info plzz" request site, and that's what many of the questions appear to be.
Based on this, I doubt that Community is growing significantly the market for Q&A sites. It may even be shrinking it! The question in my mind now is: will MyHeritage persist with the Community site? They haven't attempted to improve the site since launch, so I do wonder if MyHeritage consider the site worth the resources it costs to run.