To quote wjessop in the pregame thread, an individual substantial post is essentially any post with anything other than a vote. See below:
"If 2 players want to discuss the weather, maybe it's more substantial than initially meets the eye.
Player 1: "It's raining in Kiev this evening and I will not be able to visit my friends"
Player 2: "Oh dear, I'm sure your friends will miss you. Who would you visit if you could get to Kiev? Damned weather."
Player 1: "I'd be visiting my old uni friends but we'll have to postpone. Yeah damned weather, makes me so angry it makes me feel like killing someone. Brainbomb you're dead. Haha only kidding."
A substantial post is what it's always been: anything of substance. We're not here to decide substance; but we are here to ensure gameplay."
Further on, he made it clear that while one of these posts could be considered substantial, continuous "nothing" posts (such as saying "hi" and leaving) cannot all be considered substantial if one is to stick to the true definition of the word:
"Someone who turns up and posts variations of "blah blah blah" wouldn't be providing substance but there's also be ruining their own game, and breaking multiple other rules such as not playing to win and not playing within the spirit of the game. The rules are there to advise and encourage common sense."
As follows, to again quote wjessop, "repetition is not substantial."
So yes, Yoyo has made one substantial post today. Look at the effect it had. It might not have had the effect you wanted, but his post let you know that he's present. He introduced himself and now you're talking about him. On the same lines, continuing to post in the same manner would not be considered substantial. That's my interpretation.