It wasn't that Sinclair was smart per se, but that he was smart enough to keep even smarter people engaged.
See
http://files.righto.com/calculator/sinclair_scientific_simulator.html for a fascinating discourse on the original Sinclair Scientific calculator. And yes, that was what allowed me to put slide rule and log tables behind me. And yes, I can have the muscle memory for 3.14159265359 which is what you had to enter repeatedly if you wanted decent results.
For predecessor technology,
http://www.hp9825.com/html/osborne_s_story.html is an extremely good read. Elsewhere I've come across the story that the prototype was lent to either Hewlett or Packard for the weekend (he bred horses, and had much maths to do) and he connected the PSU the wrong way round... but there was a protective diode and fuse :-)
MarkMLl