The forum is not actually against alt accounts however the forum frowns at alt accounts.
If the forum frown at alts, I have seen many accounts that were new but you will noticed they are not new the way they are posting because they know a lot about this forum. They will keep sending merit to the account as long as the account is posting helpful posts.
Basically if you disclose your alt accounts then you are actually transparent and not trying to cheat. On the contrary those that don't publicly disclose their alt accounts are actually looking to cheat in signature campaigns and contests.
Campaign managers do not allow alts in the same campaign but most of them do not include that they do not allow alt on their another campaign. I have seen a campaign manager like Royse that allow people to apply with alt, but if two are taking, that the person should withdraw one account.
Basically if you are busy enough managing more than one account would not be easy thats why you see high ranked members with alts disclosed have their alts with low ranks.
Who tells you that ranked members disclosed their alt? No evidence. It is the only the ones that you see that you know.