This isn't an allegation, actually, but a warning. This way, scammers invite you to talk, and they might motivate you to invest somewhere. They might show fake screenshots or proof to convince you. And some innocent greedy people would fall into the trap and lose everything. So just be careful of such invitations. Rather, it's better to discuss in public places.
The higher number of persons that will make such invite are not sincere people. They likely want to drag the victim to a close chat where they hope to brainwash them into believing some false investment scheme that is going to be too good to be true and, in the process, extort the victim. As far as the forum exists, how open it is enables both the newbie investor and the learned ones to make open contributions on almost every investment discussion without the need to have such conversation in a closed door. as long as it is something that is not related to disclosing private information to forum members, there's no reason why it can't be openly talked about under a thread for others to even learn from.
Once you are invited for an investment talk in one's private message, it is either you will eventually get scammed, or you will have to pay fees to get such knowledge you would normally get for free from multiple sources on the forum; it can't be anything less than that.