They've picked a whole team of white Europeans and South Africans. As well as Teklehaimanot, they've left out Berhane, Debesay and Niyonshuti.
Cav will be under a lot of pressure to justify his inclusion with stage wins.
They have picked a team to do a job, rather than to satisfy a nationalistic/colour quota, which would have involved selecting less successful riders. Realistically they are a weak team,(arguably very weak) so no way can they support both a sprinter and a GC rider. The 4 riders you mention above have scored 0, 32, 20 and 0 World Tour ranking points to date, and we are half way through the season. By comparison guys like Haas, Fraile, Thwaites, despite doing dom work all year, EBH, Morton, and Cav, despite his illness, have significantly more points individually than the 4 Africans combined. To have gone the GC route with the African guys would have made no sense, particularly as 2 of those you mention will not have fully recovered from the Giro. Sending one or two of them to the Tour, unsupported, to compete against a load of teams packed with strong GC squads would have left them seriously exposed..
The team ethos of developing African cycling is great and to be commended, so it is understandable that these guys are revered in their own countries for putting cycling or even their country on the map, and rightly so. It is perhaps also understandable that their abilities are somewhat inflated by the local population which perhaps explains their disappointment ( see comments on team facebook) at their heroes not being in the Tour squad.
Going forward DDD have a significant problem in that 46% of their squad is African, yet very few contribute meaningful WT points. With relegation in 2019, points become critical. It is questionable whether this is a realistic number at World Tour level given the current level of African riders. If the intention is to continue at this percentage then they will need to dig deep and recruit a number of top level non African riders to secure the points until hopefully, the African riders develop further..