I mean, if they are working with Yu (which it looks like they are), then yes, they should have. Those single comments and likes, which have cost them nothing, have generated a huge amount of free PR, a shit load of articles on major news publication sites, traffic to their own website (and by proxy their other games), and have created a lot of hype for Shenmue.
If you are trying to make a buzz around something, you don't start it, and then just confirm/shut it down straight away. It makes sense to let people question it, to let the hype build and then confirm it when they are ready. Especially when they are getting all this traffic out of it.
For all we know, their first video at TGS last year with Yu could have been their first attempt at this "tease" of Shenmue IV. They could have hoped that we would go mad like we are now, but the video kind of fell a bit flat with the community as Shenmue IV wasn't directly mentioned. The original comment, that got this whole thing going, was in direct relation to that video, so that could also back up those claims.
Yet again, people seem to be getting pissed off at 110 Industries, which I don't think is valid. Yes, if in the future nothing comes of this, and it turns out they were using Shenmue IV to build their name, get free articles which they are funnelling into their other projects, then yes, hate them all you want, but at the moment, they are the best chance we have for helping Yu, and in turn Shenmue IV. Let them play this out, instead of giving them an ultimatum.
Be patient and see what happens.