This pithy little saying is unfortunately largely true. Even one plant of some of the worst weeds will produce thousands of seeds to multiply the weed load in the following year. Disturbing the soil is as good as sowing weed seeds. Weed seeds can remain dormant in the soil for decades. When the soil is dug over, the exposure to light stimulates germination.
Prevention is better than cure' .
Another irritating truism. One home grown plant from friends or family can bring with it bulbils of intractable weeds like oxalis or pieces of persisistant rhizomes such as couch grass. Striking at the right time can save time later . For example, spraying newly emerged seedling weeds will wipe out swathes of potentially large plants that are more difficult to spray later, and that take more time and more material to spray later. Hoeing on a hot day is very successful, leaving it and hoeing in showering weather is less successful because some weeds will re-root in the moist conditions and will have to be attacked again later.