It would be deferred. In your RRSP, for example, your contributions grow without incurring tax consequences. It's only when you draw down to live off the money that you pay the tax, as you draw it down.
We just want to put this on an equal footing.
Because they're holding pooled funds, when they merge, there would be a tax hit right away. You would lose your whole deferral. Let's say that you're only 65 when you retire. You might live to 95. That pool of money you have when you hit 65 is what you're going to draw down on for the rest of your life—and hopefully invest some of it to pay off the taxes as well—to keep funding your retirement. If you were in a regular mutual fund at that point, you could just draw down the money and you wouldn't have a tax hit, but because there's the pooled fund interposed, you have a tax hit.