Learn English – What does “soundness of judgment” mean

This is a standard idiom in English. The earth here is a metaphor that symbolizes the impossible, the immoderate, or the unfulfillable:

promise (someone) the earth (or moon)^NOAD
make extravagant promises to someone that are unlikely to be fulfilled:
interactive technology titillates, promises the earth, but delivers nothing

The origin of this idiom is probably from the third temptation of Jesus (in the Bible), where the devil tempts him with the ridiculous promise of all the kingdoms of the world:

Again, the devil taketh him up into an exceeding high mountain, and sheweth him all the kingdoms of the world, and the glory of them; And saith unto him, All these things will I give thee, if thou wilt fall down and worship me.
_{— Matthew 4:8–9 (The Bible, King James Version)}

This page (which is the first hit reported by a Google search on >maze bright<) makes clear that the phrase means approximately ‘capable of figuring by yourself out what you need to know and do in a new job, without formal training’:

Unless everyone is 100% Maze-Bright walking in the door and will come up-to-speed quickly on all the intricacies of stakeholder requirements, products and services, processes, organizational and job roles and responsibilities, and strategic changes needed, etc. – you might consider providing some guidance.

The author (a training consultant who has been in the field since the end of the 1970s) says he “first heard of the quality of being “maze-bright” in the early 1980s. It was an incoming requirement in most of the big Fortune 500-type organizations that I worked in.” I think it very likely that you are correct in deriving the phrase from psychological tests measuring rats' ability to find food in a maze. The earliest use of the phrase I have found is a study by W.T. Heron and B.F. Skinner, "“The rate of extinction in maze-bright and maze-dull rats”, Psychological Record, 1940, 4, 11-18.