This claim is made on the simple grounds that Gates and his company make lots of money. He was certainly a successful businessman, but that does not make him brilliant, a genius, or a visionary, any more than granny is a horse racing expert because she bet the farm on a 500-1 horse she picked blindfold, and perchance it won.
Gates in Tears in Public
A better explanation of how Microsoft made its billions is that after a some early work-a-day contracts, it suddenly found itself in unusual,anomalous and highly favourable business circumstances, riding high on IBM's shoulders, a wave of public ignorance, and undeserved brand adulation; stoked with some shady and illegal practices; supercharged by the spread of fear, uncertainty and doubt about their rivals' products; and gaining the critical mass of monopoly while the market was still small. Commerce and industry followed IBM's lead in IT at the time, and the fact that IBM had chosen Microsoft to supply DOS gave Microsoft a massive apparent endorsement, but in reality it had been an arbitrary choice for IBM at the time. For example if IBM had done a little more enquiring they could have bought DOS directly from Seattle Computer Products. In Steve Job's words, "Microsoft's orbit was made possible by a Saturn V booster called IBM".
Numerous accounts (including Paul Allen's) from within the industry indicate that Gates had none of the normal characteristics of a leader of a large company. Before his face became familiar he was often mistaken for an office junior. Instead, his leadership was based on initiating confrontations which he was prepared to take to a level of volume and nastiness to which normal people are not prepared to go, including shouted personal abuse.
Publically too, charisma, gravitas, dignity, calmness in crisis, the ability to think on his feet, all seem to be absent in him. Instead he displays obvious personality flaws including the inability to control his emotions. He is known for crying in public, tantrums, nervousness and involuntary body language. He wears celebrity like an ill-fitting suit, and if circumstances had not made him incredibly rich rather than just another computer-coding minion, his tantrums would be the only thing that people would notice about him. He has doubtless mellowed with age, but we are discussing the period of Microsoft's rise here.
Gates' deposition for the 1998 trial of Microsoft has been described as "a comic masterpiece of evasion and obfuscation". At one point, like a schoolboy trying to sound clever, he demands from the examining lawyer a definition of the word "definition". He complained later that many of the questions were off-topic, but a brilliant business man would have taken that chance to turn the tables. Instead, sullen, surly and unresponsive, he sounded like a seasoned petty crook being questioned by the police, saying repeatedly the likes of "I can't remember", "I wasn't there", "The others didn't tell me", "I didn't know", and "I didn't see anything" - even on matters of crucial company policy. A great leader would have accepted where the buck stopped and defended his company strongly; instead Gates showed his weakness of character and selfishness by practically abandoning defence of his company and defending only himself, trying to deflect blame onto others. Could this "business genius" really have been so out of touch with events at his own company, or have such a poor memory, or was he just lying? You decide.
Gates and Allen did not take the IBM contract because their "genius" could see that it would lead the company to become the richest in the World, and they the richest men. They took it because it was a job.