According to what Ash-274 has said, you're right on the money.
Well written Kirsty (edited in, forgetting my manners initially).
Very much a novice modder here, but my 2 cents.
I've made my own IV Multiplayer mod. Nothing malicious - I've made a new Cops 'n Crooks set of getaway locations. The mod is in tight distribution with trusted friends, and the title of the server is hard coded so it's clear anyone is playing a modded game mode (before I get flamed). I have enough knowledge, if I'd have wished, to do things such as enable god mode for specific players and vehicles - but I chose not to.
Open IV couldn't be used to make this kind of mod. I haven't tried to edit any GTAO scripts, but at my best guess, it would be the same story. It's not impossible of course someone might have used Open IV in conjunction with other software I'm not aware of, but from my perspective the amount of experience you'd need to do that sort of thing, you'd be fully capable of starting from scratch anyway. At most I would say Open IV might be a CONVENIENT tool for cheaters much in the same way it is for all of us (working out what R* have changed with a patch for example), but I suspect it is neither necessary nor sufficient for the trouble makers.
Arguably that's speculation on my part, tempered with experience, but here's exhibit A for the defense: the most notorious and cancerous mod menu has released a new version AFTER Open IV was shut down and the gunrunning update came out. So clearly, shutting down Open IV has not stopped cheating. I'm fairly certain the mod menus causing the problems are all written in languages like C. It might have been more plausible if they'd gone after the Script Hook as we know at least one cheat is utilising this (ripped of without AB's permission it should be noted).
In my mind there are two possibilities:
1) In response to cheating, Take2 have adopted an extreme stance banning the most major modding tool (most likely IMHO without full knowledge of it's capabilities).
2) There is another reason for shutting down Open IV, that they are not sharing. The Liberty City in V project the Open IV team were working on is one possibility, Open IV being used to inspect game files and leak pre-release info is another.
My money is on 2) and 1) being used as a (very faulty) PR smoke screen.
Which is really ridiculous when you think about it, I thought that they LOVED the leakers so much they asked the leakers to participate in their streams, and R* has said in the past that they "fondly remember" the mod that made old GTAs maps available on IV.
It just doesn't make any sense whatsoever.