To be economically literate, you need to add “…for a given wage” to that statement. Every job, without exception, would be filled for a sufficient wage. The job market is affected by supply and demand. Flooding the labour force results in lower wages, especially at the lower end of the job market.
At the heart of this argument is globalist contempt for immigrants, seeing them as little more than potential low-paid workers. This argument also allows big business to use immigrant labour as a kind of pressure valve to bypass demands for improved wages and working conditions by employing immigrants who were deceived by the ‘pot of gold’ myth, and who are therefore more willing to work in poorer conditions.
For left-wing parties to support mass immigration is the worst possible betrayal of the people they were set up to represent, the working classes.