It doesn't quite work like that.
The STI is an index, not a stock, and the index itself isn't tradable (except through futures, which are outside the scope of what we're talking about here).
But! There are stocks that track the STI - the best-known is the STI ETF, stock code ES3. (ETF, incidentally, stands for "exchange-traded fund" - it's just like a unit trust, except that you can buy and sell it whenever you want instead of having to wait for the end of the day.)
One lot of ES3 is 1,000 shares, or $3,240 at yesterday's close. 1,000 shares of the ETF is very roughly equal to the value of the STI, so you'll be close enough if you pretend that a 100-point move in the index makes you $100 per lot.
So DW's math is exactly right.
When he talks about "paper" gains, he just means "gains that you haven't crystallised yet" - you've bought a thing for $10,000, but the thing is now worth $10,300, so you have $300 of "paper" or "unrealised" gains. When you sell your thing, the $300 will become a "realised" gain. (In practice, realised and unrealised gains are the same thing - an unrealised gain is worth just as much as a realised gain - but some people treat them differently.)
