If tribe was such an issue, Somalia and TZ would be way ahead of the curve but apparently not the case. I
A implies B
is not equivalent to
not-A implies not-B The fact (or claim) that tribalism (or tribes) is the biggest problem in Africa does not logically lead to the conclusion that without tribalism (or tribe) all would be well. There may be other factors, and in the case of Tanzania one could well note some questionable economic policies.
In any case, your two examples, regardless of the actual reasons for their "failure", do not disprove the general claim on the role of tribe in Africa: they appear to be exceptions---"statistical outliers" to be "excluded"---with a preponderance of the "evidence" actually supporting the claim.
Enforcing the law of the land is paramount for any country to get anywhere. Thats what we dont do in Africa
The failure to enforce the law generally tends to be in favour of those in power, and in Africa tribe tends to play a great role in the acquisition of or access to power. As an example, right now, as Kenyans discuss who will be in power during the after the next elections, all the calculations are on the basis of tribe.