He was probably joking, but if not I do find the egos of these wealthy businesspeople something remarkable. To think that they might believe that making money and running a large for-profit organization is adequate training ground for the presidency is amazing. Now I don't necesarily think that career politicians are more "ready", but it does take a long time to learn the nuances of government, national politics, negotiating when stakes are different than just about profit, foreign governments, foreign and domestic cultures and historical relationships, and many other aspects of being the head of a 400 million-person operation in which you need to think about meeting safety/security needs, basic infrastructure for education,transportation,communication, goods, etc., economic balance/opportunity, populations that require support (poor children; elderly; mentally and phssically disabled). The ability to run a successful business is definitiely a plus -- but it's a value-added feature rather than a primary feature as far as I am concerned.
Speaking of essential features that run high on my list of importance for a president, I place maturity and emotional self-regulation high on that list. Neither Donald nor Mark seem highly skilled in those areas.