This is the opposite of what full employment means. Full employment means a fixed number of workers who mostly all have jobs already, not a fixed number of job

This is the opposite of what full employment means. Full employment means a fixed number of workers who mostly all have jobs already, not a fixed number of job opportunities. Adding more competition for labor during full employment means employers will be forced to make workers better offers, not worse offers. It does not mean every job can only be replaced by a worse job, it means the opposite of that.