Well, remember, he spoke in general terms.  His high earning royalty statements may have been in the past and he was smart with the money.  The market was much healthier when the New Teen Titans were DC's #1 selling book.  That probably paid a nice chunk of change.  He was then part of high profile crossovers, like Crisis and Infinity Gauntlet, which did huge business and have been in print in trades almost ever since.  Plus, he has done covers all over the place.  He has a lot of material that is in constant print and sells well.  Then, he developed original characters (along with Marv Wolfman) and Deathstroke is everywhere, now.  That may be providing a decent income.  There was a time when he was churning out covers, which paid well, without taking up too much time.  So, he may have made enough and socked it away that whatever royalties he gets now are enough to maintain his lifestyle.  You also have to consider he was a superstar artist and probably got more lucrative deals, after Titans was a hit, and possibly got a more favorable deal with Titans, based on his Marvel name.  It's a visual medium and an artist probably has more opportunity to make good money than a writer.  Just ask the Image crowd.