I agree, although I would add that something we should note is that French wasn't even the predominant language of colonial administration, nor is it the predominant governmental language in post-colonial Congo. The true lingua franca in both cases is and was Lingala, which was an unusual case of an African language being taught by Belgian colonial administrators to other African subjects who were previously unfamiliar with the language. Even today the concept of the Congo being a francophone country is a little misleading. It's really hard to estimate the percent of French speakers in the Congo because the only organization that has attempted to do so is the OIF which is notorious for overestimating the prevalence of French. As an aside to illustrate this point, they claim that up to 42% of Haitians can speak and understand French when virtually every other authority, including native Haitians, agree the true figure is nearly 5%. Having said that, while I'm not quite certain how many Congolese can speak French, it still occupies a position below that of Lingala (and arguably below Swahili too) when it comes to interethnic communication.
This affects the OP because a lot of the arguments against introducing Dutch were premised on saying that expecting natives to learn 3 foreign languages (Lingala, French, and Dutch) to participate in colonial administration was unrealistic. Dutch has a better fighting chance if the Belgians follow the lead of the British in Ghana or the French in Mali and simply impose European languages as local lingua francas rather than taking the unusual step of promoting Lingala as an administrative language.
While it is true that the language of contact between the Belgian administration and the Congolese population was not French, but rather Lingala (especially in the Northern and Western areas) but also, in a lesser extent, Swahili, Kikongo and Tshiluba, and that the spread of Lingala is in no small part due to its use as language of command in the army (Force publique), I don't think you can say that the language of administration as such (the paperwork, the judicial process etc.) was Lingala. Paperwork and everything behind the scenes of the Congolese administration and economy was squarely in French. (It's anecdotical, but I’ve spent some time searching through archives relating to the FP, but have only encountered French language documents.)
All this goes for both the public as well as the private sector. After all, if even a European language as Dutch, which was already co-official in the “motherland”, was deemed to be an unfit instrument of administration and other “serious stuff”, I can’t imagine the Belgian authorities granting such a status to Lingala, which most of them no doubt viewed as too much of a “primitive” language.
Certainly by the 1950s, native Congolese did have access to certain functions, both in the private as well as in the public sector. Of course, given the fundamentally racist and discriminatory nature of colonialism, these were all just low-level functions with no influence on decision-making and certainly with no possibility of being in a position of authority towards a white person (e.g. postal office clerks). Indeed, I believe Lumumba himself worked as an office clerk for Union minière for a time. In such a position, one would certainly have to be able to speak and write French in order to write reports, fill in the paperwork, etc. The addition of another language, such as Dutch, would be seen as an additional burden.
Of course, this is a factor that’s only relevant for the small number of Congolese that actually were working in such positions or wishing to do just that – the so-called évolué class and those aspiring to become an évolué. For the great majority, it would’ve been a moot point: the vast majority of Congolese by 1960 only had a limited, primary education, given to them in Lingala, Swahili or their local language by (catholic but also protestant) missionaries (so much for the
mission civilisatrice), with no or only little knowledge of French and zero perspective of playing a role in colonial administration.
But, all that being said, the Congolese matter very little when discussing the possibility of replacing French with Dutch as administrative language as proposed in the OP - the truth of the matter is that those things were decided by the Belgians, and the Belgians alone. If Dutch were even introduced in secundary education (as proposed by some in the late 50s), that would have been a Belgian decision, motivated by Belgian domestic concerns, and nothing the Congolese could've influenced.