The campaigns of Alexander happened about 400 years before the Periplus was written. So, that map depicts the known world circa the year 50 AD, but doesn't necessarily depict what trade was going on or Greek/Macedonian knowledge of the world in 330 BC.

In the Horn of Africa, there is an ongoing debate among archaeologists about what was happening in the later 1st millenium bc (i.e. 5th century-1st century bc). The debate is over what form exactly the "proto-Aksumite" period took, and when the transition from the proto-Akumite to the Aksumite period occurred. The most recent research supports the idea that the Aksumite period starts by 50 BC, where prior estimates put the transition at 150BC.^1

All of this is to say, the so-called Proto-Aksumite phase that includes the period of Alexander's conquests is poorly understood. Rodolfo Fattovich has suggested that this period saw the decline of the "Ethio-Sabean state" of Da'amat (or D'MT) in the 4th-3rd century, and resulted in a fragmentation in the Ethiopia-Eritrea highlands into petty kingdoms, out of which Aksum arose in the 1st century BC.^2 Of course, others would likely dispute Fattovich's explanation.

David Phillipson doesn't delve too deeply into speculation of what the political and diplomatic situation was. Instead he focuses on describing finds at the site of Beta Giyorgis. From the finds, he argues that foreign connections were predominately with the Nile Valley rather than with Southern Arabia.^3

TL:DR- the situation in the Ethiopian-Eritrean highlands in the 4th century BC is as clear as mud to historians. There don't seem to be the same trade contacts across the Red sea as in prior centuries. It's plausible that Alexander, or Aristotle, might not have had knowledge of an organized state in that part of the Red Sea coast.

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1 Foundations of and African Civilization: Aksum and the Northern Horn 3000 BC- AD 1270 by David W Phillipson. pp 69

2 "The Proto-Aksumite Period: An Overview" by rodolfo Fattovich and Kathryn A Bard in *Annales d'Ethiopie" 2001, vol XVII pp 3-4

3 Phillipson pp 42-44

Edit- *"Ethio-Sabaean" is used as short-hand for the notion that the temple architecture, sculpture, and script of the Da'amat kingdom is quite similar to Sabea in what is now Yemen. It is commonly accepted that such similarities reflect Sabean cultural influence on Da'amat. Fattovich is also using to contrast Da'amat culture and social organization from "Aksumite" form of social organization.