The publicity stunt that was the 1886 Emin Pasha Relief Expedition, under the then-famous (but often reviled) Henry Morton Stanley, featured one of the first portable models patented in 1884--portability being relative, as it required two people--and his popular two-volume In Darkest Africa (1890, and wow, what a title that is) about the expedition included drawings and accounts of the weapon's use. Serialized updates in newspapers during 1886, the finished books, and Stanley's somewhat teflon popularity all helped to put the weapon into the eyes of public readership eager for accounts of heroic conflict against barbaric slavers and enemies of British good governance (as they saw it). So Belloc was drawing upon an emerging popular embrace of technological power as the measure of human value and achievement, and critiquing it of course. Michael Adas gets into this in his Machines as the Measure of Men (rev ed 2014), but it was older than the Maxim, and this just happened to fit into the general arc of civilization and 'advancement' people expected. Increasingly, all sorts of mechanical power and its use served that purpose. Belloc did help to cement the image, for good and ill, but it was already being proven in first Chimurenga (Ndebele War) by Cecil Rhodes's people, British intervention in succession in Buganda (via F. D. Lugard, another larger than life figure), and a couple of other instances also in the 1890s. Such associations would prove even more important later, and they'd go way beyond the Maxim to all sorts of technologies that served colonial purposes.

As to its actual effectiveness, in open battle that's debatable, because high explosives and breech-loading repeating weapons were far more important there. It's the primacy of defense the weapon created that was arguably more important for its actual use. It was one thing to defeat a colonial population and claim to be an overlord, but it was an entirely different matter to hold that position when the same population decided they'd had enough. The massive death tolls at strongpoints during Maji Maji (the Germans in Tanganyika in 1905) and the Herero Rising (Ditto, SWA/Namibia) provide some examples, but there are many others. So the real revolution for empire was less in attack, and more in defense.