Rumba is often described as a mixture of European and African musical traditions. While probably an accurate description, I think it leaves out a lot. This sounds like a nice little collaboration with white and black musicians taking little pieces of each others music and making something new. I don't really see it going down like that. That sounds much too benign. I think more accurately rumba should be described as having compulsorily adaptated elements of European music into African traditions in order to survive.
Other African musical traditions have survived in Cuba fairly intact. Iyesa, Arara, Abakua; these musics still use African style drums and African or African based language lyrics. The element that these styles have in common is that they are more private than rumba, belonging to secret societies or requiring initiations.
Rumba is a public music. It's origins are the docks of a city, the solar of a housing tenement or the barracks of a sugar plantation. Here it was also under the eyes of the white European authority. Rumbas were periodically outlawed and it's participants jailed and prosecuted. My feeling is it eventually evolved through necessity into something less African and more palatable to the European dominant class until it was finally tolerable to the authorities.
In rumba the drums change. No longer are these African drums carved from a single tree, outlawed and banned. The instruments are pieces of a ship, cast off crates and boxes, broken barrels patched up and made into a drum, bottles, spoons, dresser drawers, etc. Junk and trash from the dominant culture scavenged by the exploited.
The older African rhythms use the lowest pitched drum for improvising and marking dancers. The new rumba changes this to the highest pitched drum. The role and function is still there, it just sounds a little different. This not only drastically changes the character of the rumba's sound from the African styles, but I have heard it described as conforming more to a European perspective of music where the higher pitched instruments tend to carry the lead in music.
Rumba lyrics are in Spanish; the authorities can understand what is being sung. The adopted form is a poetic style called decima, a venerable Spanish, having evolved in the 17th century. Rumba also adopts a device referred to as the diana, which is prevalent in the flamenco, a familiar Spanish musical form. My assumption is the European poetic styles combined with the typically non sequitur nature of rumba lyrics combine for songs that must have appeared quaint, humorous and childlike to the authorities.
So here we have a scene of oppressed and exploited workers singing nonsense songs with Spanish lyrics in a European style, playing non-African sounding rhythms with pieces of trash and refuse. Surely nothing threatening here officers, move along, move along.