A lot of assuming, speculation, bitching and moaning going on because of this tweet, and no one has a clue what it really means. He's once again triggered you lot into oblivion. That's not on accident. He vaguely threw this out, knowing you imbeciles couldn't help yourselves, that you would run wild with you Russia xenophobia, and makes fools of yourselves. This tweet literally could mean a number of different things.
First, he says they only discussed it forming one, not that they're definitely forming one. This could have been discussed, at length, as something serious or just mentioned, in passing, as part of the conversation, then forgotten about it. None of it means that one is being formed. You only assume one is definitely being formed.
Also, he never said they discussed forming it together, as a team, just that they discussed it being formed. This could have been Trump's answer to Putin saying Russia had no involvement. He could have been warning him not to try it again because the US was planning on forming an impenetrable Cyber Security unit. Again, just like you assume one is definitely being formed, you assume that it's being formed with Putin, when it could be being formed against Putin.