We can't just "skip to 6G", cellular standards are built on many of the technologies of the standards that came before. You couldn't have 5G without LTE, you won't have 6G without 5G.
China created their own 3G and 4G standards (TD-SCDMA and TD-LTE, respectively) to avoid Qualcomm's patent minefield as much as possible and stimulate the development of their own capability) I guess we could do the same and create our own flavor of 5G that tries to work around Huawei IP as much as possible.
And asking whether Trump was serious is not the right question. Asking whether he had any clue what he was talking about is the right question. And in almost all cases, the answer is no. He heard about 5G from someone and asked "well can we go to 6G right now and take the lead?" and after people tried to explain to him why that wasn't possible he probably decided that was loser talk and that's what led to his tweet. This is what happens when you have an ignoramus as a world leader.