That is categorically false. You can trade as much as you want within your TFSA, provided you don't deposit more than the annual allotment (Currently $5500 per annum).
I trade like crazy in my TFSA, there's no issues. If you refer to http://www.tfsa.gc.ca/ you will even see on the front page:
- Investment income earned in a TFSA is tax-free.
- Withdrawals from a TFSA are tax-free.
It's cut and dry with the TFSA - Anything you make in there is tax free, on the flip side any losses are not tax deductible (like an RSP) so there's the downside in the risk.