No, they are not referring to the 25th dynasty. The Afrocentrist (not pan africanist) claim is that ancient Egyptians were black; and that details of language, social organization, religion, and customs resemble modern language, social organization, customs and religion of Dogon and other peoples.

In the African Origin of Civilization by Cheikh Anta Diop (which is the ur-text of many of these claims), has an image of the Sphinx at Giza, with the following description:

1. The Sphinx, as the first French scientific mission found it in the nineteenth century. This profile is neither Greek nor Semitic: it is Bantu. Its model is said to have been Pharaoh Chephren (circa 2600 B.C., Fourth Dynasty), who built the second Giza pyramid.

Diop begins chapter 1 of his book by asking (and answering):

What were the Egyptians? In contemporary descriptions of ancient Egyptians, this question is never raised. Eyewitnesses of that period formally affirm that the Egyptians were Blacks. On several occasions Herodotus insists on the Negro character of the Egyptians and even uses this for indirect demonstrations.....Herodotus says "the Egyptians said that they believed the Colchians to be descended from the army of Sesostris. My own conjectures were founded, first, on the fact that they are black skinned and have woolly hair ...."

Cheikh Anta Diop is being extremely clear in his meaning that all ancient Egyptians, not only the 25th dynasty, were black.

To explain the appearance of modern Egyptians, Joseph Ben Jochannan argued in Black Man of the Nile that the Arab Conquests in the 7th century resulted in "cultural destruction" and "Arab colonization" of Egypt, and implies that this resulted in large-scale population changes, resulting in the dominance of the modern Arab population of Egypt.

Molefi Kete Asante describes a similar scenario of successive waves of conquests changing the population of Egypt. (from The History of Africa; the quest for eternal harmony):

The Color Problem in Contemporary Egypt The overwhelming majority of modern Egyptians are descendants of Arabs who began arriving in the country during the 7th century CE. However, the ancient Egyptians, those responsible for the construction of what we know as monumental Egypt, were “black skinned people with wooly hair” (Herodotus 2003). Of course the ancient Egyptians, even at the time of Arab entry, had already seen some amalgamation with invaders such as Assyrians, Persians, and Greeks. Over time, and with increasing migrations during the current era, people arrived in Egypt from Albania, Russia, and Turkey to join the indigenous Egyptians, Arabs, and mixed populations in the country. Yet the black population, laying claim to its ancient history, remains a significant part of the country until this day.

I want to underscore a particular line. "The black population, laying claim to its ancient history..." The implicit framing by Cheikh Anta Diop, Joseph Ben Jochannan, and Molefi Kete Asante is that the ancient history of Egypt belongs to black Africans, and specifically belongs to Nubian populations in modern Egypt. As a corollary, this framing denies heritage connection of "Arab Egyptians" to the Pharonic history of Egypt. Further, Afrocentrists have claimed that European scholars have tried and continue to try to falsify evidence in order to "give" Ancient Egyptian history to Arab/semitic peoples and deny it to Black peoples.

Non-Afrocentrists agree with portions of Molefi Kete Asante's quote above. It is true, and not controversial, that there have been population flows of Greeks, Arabs, Persians, Russians, Turks, Romans, into Egypt. However, modern Egyptologists argue that even before Assyrian or Persian invasions, the population of Pharonic Egypt was not homogeneous. There have always been population flows into the Nile valley, from the south as well as from the Levant and from Libya. And furthermore, modern Egyptologists reject the "population replacement" model of Ben Jochannan and the soft-population replacement of Asante. They stress the admixture of Arab populations into Egyptian populations, meaning that modern "Arab Egyptians" can trace their ancestry back to Pharonic era Egyptians.