Is sour milk, soured milk, and milk that has gone sour, all the exact same thing?
Yes, it is the same. It refers to milk which has been left out until it has gone sour with whatever wild bacteria it has managed to catch, be they pathogenic, healthy, or neutral. It curdles a bit and changes in smell and taste.
If you find a person or author who makes a difference, then you have discovered either some regionally restricted distinction which is not widespread (maybe even restricted to that one author), or a too-literal translation from some other language.

Update, since you seem to be asking about the exact meaning of Stephie's comment. What she means is that back in the day, when milk went sour, the actual risks were lower than when today's milk goes sour. (not to speak of the perceived risks and of the implicit risks/reward calculations people make by gut feeling). The difference in her comment is not between &quot;sour milk&quot; and &quot;soured milk&quot;, but between &quot;milk that was freshly milked from a single healthy cow and left at room temperature until it went sour&quot; and &quot;commercially produced and processed milk which was distributed to me and left in the fridge until it went sour&quot;. There are no terms in the English language which make this distinction, both are called &quot;sour milk&quot; or &quot;soured milk&quot; - so the product is probabilistically different, but the words are not.