The difference is that an innocuous comment can (and inevitably will, especially since Go apparently does not actually care about comments, only that a line starts with //go:[0]) match the directive syntax at some point and then it's a right pain in the ass to debug, if that doesn't break anything.
I think they should fix that bug where it's not actually looking at the comments. However, with the exception of that bug, I don't think I have ever made a comment that started with "//go:" and was not a directive. It seems kind of hard to make one of those by accident.
[0] https://news.ycombinator.com/item?id=9523187