Maybe we have different definitions of the word fact but I'd suggest what you are saying is actually a hypothetical and not a fact at all.
Ethiopia was never colonised and look how they turned out compared to the rest of Africa. It's very tempting to try simplify these highly complex realities, the relative comparison you've made doesn't hold water though.
I stand by my statement: they'd be better off left to their own fate. But maybe you're right, it's not factual.
Most resource-oriented economies (oil states, Brazil, Australia, Norway etc) are extraction-oriented and this doesn't seem related to colonialism. "Dutch disease" is named after the over reliance the Dutch developed on their extraction-oriented economic sector - it's a very hard thing to avoid.
Sure, when oil extraction started, Brazil has not been a colony for a century but the extraction culture was there.
Still, without colonialism, Brazil wouldn't be a thing, there was pretty much no commerce there, unlike with some african areas
Of course Brazil, Australia and Norway export their goods, but they also build other infra needed by their citizens. Colonizers, not so much.
And for it to be sustainable the end user needs to be able to get a gain from it (even if given some fantastic hypothetical world like being robots who depend upon crude oil to survive instead of food).
Essentially the combination of the two may be technically peripheral in that competive advantage results in a dutch diseased country and corruption leads to lack of investment in infastructure beyond what is needed. Thus colonial extractive economy is one cause but it may occur without any colonalism per se - just as a result of internal actions. Lets call this manifestation "pseudocolonial". The only involvement of the foreign in the pseudocolonial is a willingness to trade. One may assign guilt or morality in another nation for choosing to accept trade or boycott or embargo but either is just as much intervention.
To be pedantic I suspect the framework may be conflating aspects - this isn't apologetics towards actual horrific extractive colonies which short sightedly squander human potential and cause needless suffering.
I think there a definitely places in Africa doing better than it but also plenty of places doing worse.
What's your view on Ethiopia
The famine in the 1980s was disastrous, and then there were periods of civil war and war with Eritrea which lasted until the early 2000s. Ethiopia is landlocked, and the wars as well as civil wars in both Somalia and Sudan made trade very difficult.
But the economy has been growing well since then, and it is far from a failed state like neighbouring Somalia or Sudan.
I'm not really sure a single example like that can be projected to a pattern.
The state of Ethiopia isn't so far different to other places, but that's mostly because after colonialism ended too many countries slipped backwards into the sort of tribal warfare, Marxism, corruption, economic mismanagement etc that typified the non-colonialist states. Do you think the British would have tolerated a Marxist insurrection in a colony, if they'd still been in charge?
The article alludes to this when it talks about how the illiterate man would prefer being ruled by a white man, because he thinks he'd be treated more fairly. Colonialism created many problems, but the state of these countries after it ended puts it in perspective.