I'm afraid the idea that most trans people pass so well that nobody can tell is a bit of an urban myth in the trans community. You often see on trans subreddits posters claiming that cis people are so dumb they're incapable of telling that someone is trans.
But it's not true. In my whole life, I have known one transwoman who passed well - not perfectly, not if you looked closely, but well enough that on a brief acquaintance you'd probably take them for a tallish woman with a husky voice. Not necessarily though, because some people are more alert to subtle sex differences.
With all the others I've known, I'm afraid their trans status stood out a mile. I take no pleasure in saying that, because it feels a bit like saying your friends are ugly, but they didn't remotely pass, they knew they didn't pass, and everyone was just being polite.
It's the same with the mythical 6-foot burly transman we keep hearing about. She's a statistical possibility because tall and burly women exist, but every transman I've met has been on the short side even for a woman, and the facial hair does all the heavy lifting. You can tell by the voice, the gait, the narrow shoulders and wide hips, the mannerisms.
So it's not just that someone's philosophical belief about their identity doesn't match the material reality of their sexed body, it's that the incongruence between their philosophical belief and their sexed body is readily visible to almost everyone they meet.
This is relevant to safeguarding because of the wildly different offending patterns between the sexes, and because a biologically male person deciding to identify as female does not thereby acquire the risk profile of a biological female.