Correct, it was repealed this year, but was the prevailing law previously.
As I said, I'm not an accountant nor a lawyer, and yes, AFIP is more interested in getting people pay something via being monotribuistas than be fully en negro, but the issue I'd caution you and others with, which your accountant may or may not have told you, is that you are making fines, intimations, and even criminal prosecution very easy for AFIP and the UIF should they ever decide to actually enforce the laws (even years from now, and I've been reading cases in which they do this) or if you get connected to someone/their CUIT/L that is investigated for violation.
For example, you've created a direct paper/electronic trail demonstrating that you violated MULC regulations, falsified invoices (Factura "E" are not supposed to be issued in the manor you described), violated the regulations of the Monotribuista regime itself, and also violated WesternUnion's end user agreement for Argentina, including rules imposed on them by AFIP/UIF itself, and these are just to name a few.
Now, all that being said, I'm not suggesting you should panic or fire your accountant tomorrow morning or brace for a knock at your door by the UIF; my objective isn't to scare you or Shpongle or others; I honestly don't condone or condemn these kinds of practices in Argentina and am well aware that virtually no tech workers are fully en blanco, but I do think it's like recreational drug use or gambling: you should know the laws, the potential consequences of engaging in these practices, and review your tolerance for risk. Do I think you'll ever get caught? I'm a gambling man, and I wouldn't bet on it, but you've made it easier for them to throw the book at you if you do.
Hopefully once the cepo goes away so will the requirement to settle via the MULC and people can simply issue Factura E invoices and pay their taxes accordingly without the need for every bank to get a cut, and workers to lose 20% of their salary to the distortions caused by capital controls.