To some degree depends on how much Hou is metrics-driven vs scouting/x and o driven in their decisions. We know that they have a ton of metrics and are very sophisticated about using them, but I don't think we necessarily know degree to which they consider other things (is this the current Red Sox decision making structure, with a significant scouting input, or something else?). Rondo and Parsons are two guys that tend to polarize those different approaches in terms of value.
I'd be pretty surprised if that specific proposal was real; I would not be surprised if Houston decided Rondo is worth more to them than Parsons, though. I'm not sure that I see how the rest of a deal would come together that both sides would want to do (I think Celts would want more, not sure Hou would want to give much more, and also hard to see how other roster pieces end up in right places)
As to fit, I think Houston probably feels a penetrator and distributor is a real gap. They are pretty dependent on Harden creating for them in half-court, and I'm not sure they can win in Western playoffs without someone else creating offense. They also may feel they need a better on-the-ball PG than Lin to play OKC and SA (though Rondo is hit or miss there himself, depending on motivation). They definitely would miss Parsons; I'd imagine they would plan to backfill that with another shooter, because I don't see a pure fit on their roster.