LOC against PPOR – interest deductibility confusion

Hi people

I've just read page 72 of the April 2011 edition of "Your Investment Property" magazine, and am now confused.  It is talking about a family with a PPOR mortgage, and how to speed up paying it off.  Yes they have surplus income they can throw at, fine.  Here is what confuses me:

The article suggests that the family could set up a line of credit against the PPOR mortage, and pump all spare money into that.  Fine.  Happy with that bit.  It then goes on to say:

"If he's paying 7.5% or 8% interest on his loan, then repaying that quicker is like earning 10% interest on his money because by the time you earn 10% interest and pay 30% of it in tax, you have 7% left over.  But if you're paying off debt there's no tax involved, so it's like getting 10% on your money"

Here's the thing though:  surely the interest on the LOC would not be deductible, because it is against the home loan and set up for personal (non-investment) expenses?

If I am wrong and there is some sneaky way to make the interest on a PPOR loan deductible, brilliant.  But I presume not.  Could those in the know help me understand what this article might have been talking about?

Thankyou!

JacM