how Guinea is *really* part of Mali (since it was part of the empire at one time).
it's even more complicated. We can't say Guinea was part of Mali once and is not today. Today's Mali is not that directly historically related to the Mali (or Melli) empire in the 13th-15th century. Or in other words: Guinea could have taken the name of Mali as well. The biggest Maninka town of 1958/60 was Kankan, Sundyata's capital was Niani - both in the territory of the odern state Guinea.
At the same time, the modern state République du Mali is dominated by a 25% Bambara population, not by a Maninka population (Bambara was a little vasal of the Mali empire and became two slave catching and trading kingdoms in the 17th or 18th century). The Republic of Mali was founded after a political clash between the groups of Modibo Keita (Maninka) and Léopold Senghor (Wolof, wasn't he?) in July or August 1960 about the question of who (and how) should lead the new built "Mali Federation" (capital: Dakar, in a region that was not or not directly part of the Mali empire).
They had been looking for a "great historical" name for their federation. And as there were only 3 possibilities, and Songhai or Gao was ethnically too related to the Songhay population (and the french colonies of Sudan and Senegal to much in the west) and Ghana was already taken (strange idea: the Gana empire had not reached the Gold Coast), they took "Mali". In the end, the guys in Dakar decided to make a France related state without there semi-socialist "friends" in Bamako and to keep the colony's name (Senegal). So they called the Bamako state with half of the desert (where Maninka and Bambara only had come through as slaves before) "Mali".