Honestly, I find it hard to believe that with just this scheme (have 1 or more alts) the account will level up.
That is just one scheme, a naive one. What is more likely (among other things) is that they have managed to get one or several of their colleagues promoted to merit sources which now support the farming. Do you not read stories of organized scam centers in third world countries? That is how it works. They have groups and do this together. Even if all the accounts involved are not yours it is still abuse. Further, you can make it seem more legitimate by having a strict rule of not giving merit to your own accounts across the group. In that case they would only give merit to the accounts of their conspirators and never to their own account.
You earlier mentioned subjectivity as an argument, which I strongly reject. The problem with subjectivity is that they can and already are using it to conceal their organized farming. One example: Make nice artwork thread similar to the Pizza Day contest in a local section as a merit source, and you can give a lot of merit to all the people in the farming group at the same time without it looking too suspicious. Subjectivity has its place, but the intent is what matters. If merits are easily earned in a local section for low effort contributions, after which the user proceeds to shitpost in other sections for campaign money, the intent is clear from the start.
Of course this is extremely hard to prove compared to direct alt abuses, but you can spot some patterns because of the low quality of posts. Like I said previously, a system that rewards quality contributions must over time naturally increase the average quality of the contributions assuming that it works. According to mindrust there was a period in which this was the case, and this empirically supports this argument. It seems that the system needs new tweaking to function again. I always through that a demerit function within such systems would be nice, but it requires more oversight as it can leads to gang behavior and significant abuse.