Even after these guns got replaced for good in military arsenals, even reservists getting more modern gear, some armies kept them around in storage. The Brazilian Army was one of those, eventually they were transferred over to law enforcement agencies, such as those in Rio de Janeiro, to name just some, at some point they are converted from 7x57mm Mauser to the then more common 7.62x51mm NATO cartridge.
Famously, these old world machineguns has actually seen rather heavy use by Brazilian police forces for more intense urban combat. Though officially, these old warhorses were supposed to have been retired in 2008, they never actually stopped using them, here is footage from a firefight in Barão back in 2019.
https://youtu.be/1WG7GDnqC7E?t=734For why they're being used, it's probably as simple as the fact that these machineguns were A), free, and B), there's not that much budget for these departments, and C), they still work absolutely fine. When units like these may have to fight gangs of very hardened and well armed criminals, there's some appeal to a dead reliable and powerful machinegun, a gun which just doesn't stop and never wears out.
You can be as high as the sky, fearing nothing, feeling no pain, but there's really no arguing with M80 Ball, a 147gr .308 caliber FMJ projectile going 2800fps pulverizes your hip bone or spinal column, you're out of the game, and then the other bullets in that burst of fire penetrating the car your thug pals were hiding behind, opening up brain cavities and pulping hearts.
I think you kind of have to be a hard motherfucker to get into firefights with cartel members, using a heavy and antique machinegun like this as if it's a rifle, particularly in the case of the linked video, where the guy in question seems to only have the one magazine which he has to refill with loose cartridges at times. All in the 21st century.