The highest tax rate payable in Alberta is 39%.
If you were to make 9.34% in interest you'd in theory pay 39% tax on that (leaving you with an after-tax gain of approximately 5.7%). Of course, his math doesn't account for the fact that you can shelter your investments through RRSPs and TFSAs or that capital gains are only taxed at 50% and Dividends at 25% or that inflation can make deferring mortgage payments a smart move (assuming you're investing in something that is inflation defensive) etc. etc. etc.