Totally agree (well maybe I would be more lenient about the rating). I also played Calendra's Cistern 20 years ago and replayed it again. The City is massive and quite nicely designed. Just like you, I was surprised with a linear walkthrough requiring you to unlock further parts of map. This approach feels quite strange now, especially after I played FMs from Skacky, Nicked or Korthak. But again, their stuff is made a decade later.
While the mission's size can be explained by the fact that it was built for the Summer Speedbuild Competition, I think it's completely fine as it is (perhaps the outside area could be more detailed, but that's all). I wish more fm authors would consider building small sized missions (which you can finish under 1 hour), not everything has to be an epic 2-3 hours intensive thievery. I sometimes mind a bit that the more experienced and talented creators avoid thinking in smaller scale, but cons...