No, it was the right question in querying your evaluations. If I could talk to Olshey or Cho or Pritchett directly, I'd ask different questions.
That isn't how things work in real life. You don't simply "know" or "not know." Every evaluation is probabilistic. You don't sign Lopez or Davis because you either know for a fact that they're going to be really good or else just sign at random. Similarly, you don't either draft Lillard because you know for a fact he'll be really good or else just draft randomly. All signings and draft picks are based on a probabilistic evaluation--you use your evaluation and analytical skills to determine the best risks and then go after those guys and hope (yes, hope) that those risks pan out. Every single player you acquire is a risk proposal, though you can reduce the risks at higher costs (signing LeBron James carries far lower risk at far higher price).
Saying that "hope is not a plan" is a good bumper sticker but a poor reflection of how any kind of business gets done in the real world. Yes, you don't want to rely on hope, but after you've done the work and (ideally) made smart decisions based on the existing evidence, all you can do is hope that smart investments actually pan out. Taking a bunch of 75% success rate bets at low cost, for example, is smart but there's a 25% chance on each of them that they'll still fail.
This is obviously complicated by the fact that no one on Earth knows the actual chances of any particular player panning out.
So no, the Blazers didn't know what they were getting. That's the wrong question to ask. The right question to ask is, what makes you bet on one player over another? In the case of Davis and Plumlee, it's that they're proven strong rebounders (based on what percentage of rebounding opportunities they corral) and have shown good explosion and finishing ability in the pick-and-roll, which is a big deal in what is currently a pick-and-roll league. In addition, they've both shown production or potential as rim protectors (which is the most desired defensive ability in a big man these days). In Plumlee's case, he followed a strong rookie season with an up-and-down second season. So, obviously there's some risk here, but there's a lot of upside reward.