The comparison still doesn't work.
The only prerequisite to get a parser is to have a pc. It costs nothing but hard drive space. It costs zero gil or real cash. You don't even need to raid or play with anyone else to use it. There is no skill threshold for using a parser. You can be the worst player in the world and still use one. You can use a parser as much as you like and if you tell no one you have it, no one will know that you do. This is also applicable to mods that edit how the game is displayed to only the player, such as reshader mods.
If you want to sell boss kills you first need to be a very skilled player who is part of a very skilled raid team who can reliably kill high end bosses with little trouble. An immense amount of time goes into becoming that sort of raid team. Most players don't have the time, the skill and the team to do that. So many things must align for killing high end bosses with relative ease to be a possibility. Even if everyone in the game wanted to sell high end boss kills, most of them will never have any to sell.
On top of that the entire team must agree to engage in RMT. If one doesn't, that's it the plan is on hold. And on top of that they need to find willing customers, and most players either prefer to earn something themselves or don't have several hundred dollars lying around doing nothing. Furthermore this cannot be kept secret because if you have something to sell then you need to advertise.
While both are against the terms of use, they're both radically different problems. One is just free software displaying information the game already has. You can't actually say it's illegal, it's merely against SE's terms of use. The other is basically selling content that belongs to SE for real cash when you are not SE. Selling boss kills is pretty much having your own Mog Station but with different products. It is against SE's terms of use and also most likely very illegal.
I understand your slippery slope argument but it's simply not applicable to selling boss kills. I strongly disagree with it, but I also understand that only a tiny percentage of players can even hope to be able to do it. A slippery slope can only happen if the majority of players can participate. This is not the case for selling or buying boss kills.
Again I emphasise that I agree that SE should take action against selling boss kills. I just disagree that the issue is comparable to parsing.