This is from the author's perspective, which just feels wrong. It doesn't feel natural. I cannot believe anyone would feel 'caught out' by the student-professor exchange. The scope of the project was a little vague, and the research team would just shape it as it went along.

The 'gotcha' moment just doesn't exist here; the awkwardness sounds like something that was artificially perceived on the student's side. In any usual situation, the professor would say... "sure, but we're going to do it anyway" or "sure, but we'll expand scope" or, more likely yet, "sure, but we'll work it out as we go along".

With respect to the retraction of the offer, it is more likely to be because either: 1) the research team failed to organise bits and pieces in time, shifting priorities, or just didn't fully commit to the project in the first place; or, and I hate to say this, 2) the exchange revealed that the student was a little socially awkward, which they took as a red flag.