For people who don't have mortgages the investment advice is still relevant. You just toss out the mortgage part and anything related to the home ownership aspect such as HELOC's in the USA or other equivalents (I don't know if they are different in other countries).
But the basics of that article are still sound. Put money away into a retirement investment vehicle (old person money as some people call it, and the article calls for 20% on this), put money into other investments (in the articles case 20% into what they call aggressive share investment, we'd probably call index fund investing), and the 50% into mortgage just gets translated into pay your rent (any difference between rent and 50% can be invested in the prior two mentioned or put aside for a future mortgage or whatever you want).