I’ve seen MB described as “descriptive, rather than prescriptive,” and I think that sums it up well.
Myers-Briggs presumes that we each possess all eight of these functions, and so can “be” any given type in different circumstances; it’s a matter of which functions we prefer most/are most dominant. And they can change. See, again: polarity. I’m certainly not saying that INFJs are the only ones who have moral crusades or righteous anger.
“Typing” historical figures (and sometimes even yourself) is always subjective, of course. The folks at PersonalityCafe, for example, have been debating Jesus as ENFJ/INFJ/INFP for years.
Followers of Jung, you can imagine, tend to regard aspects of psychology as artform — which bolsters the modern psychology community’s belief that Jung’s theories are “religion”, despite their scientific origins.
That Hitler and Jesus and I all share the same type is not, however, the point of my conclusion, so I don’t think it much matters. It’s just a tool.