umat does not test work ethic or ur ability in science (this is looked at thru your ATAR and academic transcript). so some1 whos never studied science in their life could potentially do very well on the umat.
however, it does test ur ability in other areas. e.g. how well you understand people and how well you can understand and retain information. the former is taught and learned throughout the course while the latter is required from the start. that is why UMelb placed double weighting on s1.
conclusion: the umat tests for some of the abilities required for medicine, but not all. therefore if you do well on the umat, it does not neccessarily mean you will succeed in the course.
(i would've certainly hoped there was a correlation, but i doubt it).