I don’t believe that agriculture only started then. It’s just the earliest evidence we have. Everything always gets pushed earlier
Yeah, you can do a lot of agriculture just by planting some plants and comming back later which would have looked basically the same as hunter gathering to archeologists. Similar stuff is done to this day for cannabis https://www.vice.com/en/article/meet-the-gangster-free-weed-...
It doesn't actually look the same to archaeologists. One of the things we'll do is look for subtle changes in seed morphology as a sign of domestication. There are also methods of seeing what types of plants are growing in a region, which changes when humans begin selectively cultivating certain plants. We can also get a rough estimate of how many people were in an area and in what seasons.