If you have been thinking about joining TC, but never quite got round to it, there would seem to be a brief window of opportunity to open an account. It would be best to get it done before Christmas, though. I will be surprised if they process applications between Christmas and the new year.
The announcement does not say that you need to have actually invested funds before the new year .
Lender fees are tax inefficient for individuals, and they could have raised the money by increasing borrower fees instead. I think they have done it this way for two reasons: (1) to exempt existing lenders, and (2) more generally, to support the possibility of different fees for different types of lender, eg discounts for institutional lenders.
I would also infer that TC see their future growth being mainly in a combination of (1) institutional lending and (2) individual lending via tax free IFISAs.