OK, I have looked at forum rules and seem to not be finding some that are enforced. Current example is my posting an item for sale that I also have in GE considerably below the recommended price. There is no way for someone to know looking at the ge that an item is available at lower prices without bidding. I see no rule saying I can't post in trade forums if posting in ge, yet the post got locked and review says handled correctly. Is trade being limited even more again? Thought that ended when the ge got revamped.
That rule has been in the MarketPlace Forum for the past 10 years.
Those Forums are there to arrange direct trades with other players. They are not to advertise that you have an offer in the GE, as there is no way for anyone to know whose item they are buying or who is buying their item.
The "you don't need to make threads for items that you're selling on the Grand Exchange" wording is a nice way of saying that advertising GE offers on the Forums is not necessary and therefore is spam (which is not allowed).
As I said I couldn't find it and I did do search, neither GE or grand exchange or forum rules returned that link but thankfully Amaterasu Mi (thanks again, had to scroll up for spelling) did. No idea what it being in rules for 10 years or not has to do with anything. I won't be posting like that in the future, but stating that something is not needed is a far cry from not allowed and fits no definition of spam I have ever seen. As the previous rules in that same post should be clarified as they sound like the mentioned events are allowed just discouraged. Things can be said nicely without losing clarity.
Seems pointless to me as you can just remake it saying selling it for x and not mention it is also on the ge and then if it doesn't sell before you get a pm then you just take it out of ge and sell by trade...