I got the clue, but as you also noted that it would be possible only if the user received initial merit airdrop.
I did not state that. Not all farmers are shit posters, some contribute to the forum constructively and get merits for that, they can now circulate that merit around their several alt accounts .
Again, I think the merit farm will only work for a merit source ad you noted. Someone that has 100+ activities but below 50 merits will find it more difficult to come up with quality posts if created another account. Infact the more account he creates the more shit his posts will become.
Being a merit source would definitely be an added advantage, but it is not the only scenario this would work in.
Also, farms are not always run by the same person, it could be done by a group.