how accurate is he?

Historically, not very. Writing emerged in a number of different cultures during the Bronze Age, having developed from various proto-writing scripts of the Neolithic. So it isn't accurate to say that writing started in Sumer or Egypt, because writing appears to have developed independently in a number of different cultures around the same time (or at least, the surviving artifacts are from around the same time). However, in The First Writing: Script Invention as History and Process Bruce G Trigger writes:

As late as the 1960s, it was widely assumed that all early scripts probably were derived from the Sumerian writing system that had developed in southern Iraq beginning in the late fourth millennium DC. The fact that this script appeared to have taken longer to devleop than any other was interpreted as evidence that it had evolved locally [...]. It was argued that knowledge of writing had diffused along with various other cultural traits from Sumer to Elam, Egypt, the Indus Valley, and eventually Northern China, giving rise to the oldest scripts in each of those regions. The geographically remote Maya script, which was then undeciphered, was debarred from consideration by alleging that it was merely an example of semasiography, not writing [...] This diffusionist scenario corresponded with more general Eurocentric beliefs that, while Western civilization had begun in the Middle East, it had been perfected in Europe.

Which is where your video comes in. Bias on the part of historians, linguists, archaeologists, etc. is a real and acknowledged issue, especially when it comes to the interpretation of finds and data, and in previous decades many predominantly Western/European scholars held to diffusionist models (interest in the Middle East also went hand-in-hand with Biblical studies, naturally enough).

Politics also has its role, and that's sort of the problem with this video: pointing out that there is (or at least, has been) a pro-European/Biblical bias to the study and interpretation of early writing is fine. There are plenty of examples of that, like Ignace J. Gelb's A Study of Writing, which tries to make the connection that Trigger was talking about above.

The African/Ancient Kemet angle however is a slant toward historical revisionism, you see this kind of thing come up with articles about the Black King of Scotland and the like, where people looking to push a pro-African version of history seek to reinterpret the evidence in favor of their pet theory.

Which doesn't mean that writing in Sumer might not have had an African inspiration! It just means that currently, there's not sufficient evidence to suggest such an African origin for Sumerian writing systems.