How was this law applied to those who already had jobs?
Have no idea.
It sounds like that was answered already: if they were poor enough to qualify, in spite of having a job, so long as it was at least a 20 hour a week job, they stayed on the roles, if it was less, they had to top it up with job training.
The article said it was able-bodied people who were not working.
One would assume that, the real issue being how to delineate need vs. convenience, those who are actually working and still have a valid need will not be dumped. Especially since the prerequisite to actually show a willingness to be useful to society and earn the benefits seems to be the standard. I have always been a proponent of staging folks off the dole by allowing them to work and taper off the benefits in a way that keeps them a bit ahead of the game. If they go from zero income to $100 a week, drop the benefits by $50 a week. It provides incentive for those who are actually willing to try to become self-sufficient better than having a specific cutoff where one gets further behind by working.
I can see why the Left hates it - they depend on folks staying reliant on the governmenthard working taxpayers. When more people discover that it is an abomination to rob one set of folks to support the dregs, the Dems lose votes.