The article itself admits that "Africa’s Swahili-speaking zone now extends across a full third of the continent." That excludes two-thirds of the continent.
For example, in South Africa, there are a whopping eleven official languages, including Xhosa and Zulu (both so-called "Bantu" languages like Swahili), but Swahili is not one of them.
The same is true of most other Southern African countries: Angola, Botswana, Lesotho, Mozambique, Namibia, Swaziland, Zambia, Zimbabwe.
As another example, Wikipedia has a list of 46 (!) languages spoken in Angola: https://en.wikipedia.org/wiki/Languages_of_Angola . Swahili is not included.
There has been an effort to begin teaching Swahili in schools in both South Africa and Botswana, at least. But such efforts don't have a good track record of success. Take a look at Ireland, where students are taught the Irish language in school but, for the most part, are terrible at it. It seems unlikely that imposing a language on countries that don't natively speak it will succeed.