The term “diacritic” has a specific mening. It means a symbol added to a letter to alter or disambiguate its sound. The dot in i and j does not have that function (in English), therefore they are not diacritics but part of the letter itself.
The dot evolved to make the letter more visually distinct in handwriting, not to indicate a sound change.
The dot evolved to make the letter more visually distinct in handwriting, not to indicate a sound change.