Look, it should be obvious what this really means. It means that Trump is telling Republican primary voters what they want to hear instead of telling the Republican donor class what it wants to hear. When Republican primary voters say that Trump is saying what he believes, what they really mean is that he is saying what I believe. And if I believe it, how could anyone be insincere in saying it? That is, of course, a perennial danger -- that people want to believe other people who tell them what they want to hear and often have great difficulty detecting even the most obvious liar and con man if he knows just what to say.
After all, as the saying goes, the lie can travel halfway round the world while truth is still putting its boots on. Why this is so is straightforward enough. A lie can be made as pithy, as memorable, as simple, and as emotionally satisfying as a good con man can make it. The truth is under no such obligation. Quite the contrary, the truth is usually subtle, nuanced, confusing, and not well matched to anybody's preconception. The person who tries to counter a well-crafted lie with a messy truth, or a simple lie with a nuanced truth, just comes across as being evasive or mealy-mouthed.
It is easy for Trump's opponents to see that he is not being authentic but merely pandering because he isn't pandering to them. To people hearing their own beliefs presented as challenges to some despised elite, it comes across as fearless truth-telling -- the "truths" they have wanted to hear for a long time.