Some quick math assuming each precinct is about the same size - about 15,000 uncounted votes in Fulton County and about 1,700 uncounted votes in DeKalb County. The President is right that they cannot overcome 100,000+ votes lead.
You just prompted another thought: While the media are roasting Trump for his speech, I think it was smart to get out there immediately and let everyone know what the numbers are, and that it is nearly mathematically impossible for Biden to overcome them. Then, if the dems continue to stall and try to manufacture additional votes, it will hopefully make it clearer what is really happening.