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That's not actually true in this case. The author is correct about this.

When "int i = 10;" is encountered, the tentative definition behaves effectively as a declaration. If the compiler were to reach the end of the translation unit and the variable i was never defined elsewhere, however, "int i;" serves as a definition.

In think it is the other way around - the declaration is a tentative definition, so a definition unless there is a definition at file scope.

Isn't a declaration "extern int i;"? Without the "extern", it's a definition, there's a symbol in that translation unit, etc.

No, it has nothing do with scope qualifiers. This is the same mechanism that used e.g. to declare recurrent structures.

Indeed. I guess his title is correct for him, at least.