Well firstly, why has Belfort got such low testosterone? Even if he did need TRT, and that's a big if, it's because he's abused steroids in the past meaning his testosterone levels are all out of whack. So he got to cheat throughout his early career and then in the latter part of his career he gets to use TRT because he cheated? That's not fair for a start.
Secondly, you have no idea how much TRT he is taking. No one knows what his levels were, he may have been well out of the 6:1 ratio. Two of his last fights were in Brazil.
Thirdly, of course it gives you an advantage! He gets to have 17 years experience of competing in MMA and he gets to have the testosterone levels of a younger fighter? How is that fair? Particularly when the only reason his levels are so low is because he cheated in the first place with steroids. It's grossly unfair. Not only that but some of these guys on TRT are going well above the 6:1 ratio in training which means they recover faster and then they're cycling off it to get past the commission. Vitor hasn't even needed to do that since his last two fights were in Brazil.
And of course extra testosterone gives an athlete the ability to perform much higher than standard, why else go above the levels? That spinning heel kick requires explosiveness to do it, which can be provided from the use of steroids. It was cool as feck though and a highly skilled move, maybe he still would have pulled it off even if he wasn't on TRT but he is, and since he has a past of abusing steroids then I don't see it as fair.