Starch is produced by all green plants as an energy store. Amylase production is also a result of fruit and veg consumption.
It doesn't actually prove we ate tubers.
Yams of African species must be cooked to be safely eaten, because various natural substances in raw yams can cause illness if consumed. (Excessive skin contact with uncooked yam fluids can cause the skin to itch. If this occurs, a quick cold bath will stop the itching.) Preparing these species is a time-consuming process, involving several minutes of pounding, leaching, and boiling to remove the toxins.
There isn't any evidence that Grok was boiling water (or leeching pulped foods), so safely consuming yam may not have been possible.
Tubers may have only been a 'last resort' food source
The "Seven Deadly Sins"
Grains (wheat/rice/oats etc) . . . . . Dairy (milk/yogurt/butter/cheese etc) . . . . . Nightshades (peppers/tomato/eggplant etc)
Tubers (potato/arrowroot etc) . . . Modernly palatable (cashews/olives etc) . . . Refined foods (salt/sugars etc )
Legumes (soy/beans/peas etc)