This is not true in every instance, because a lot of these scammers has caught on to the fact that short discussions are scrutinized more often and easily identified, so they are writing more detailed threads lately
You might have to change that statement a little and probably say, "The OP is mostly very short". If people are only cautious of short threads, then they will fall for the longer and more detailed threads.
I'm not claiming that any of the red flags that I've posted is enough to call someone a spammer, even all of them at once don't necessarily mean that it's a spam, those are just things that should raise your suspicion. You're right that spammers can have longer posts, and you are right about poor grammar, I even considered adding grammar as another red flag, but decided against it, because there's a lot of non-native speakers on this forum, so it's not a strong reason for suspicion.