You will be less likely to miss your flight arriving 4 hours early instead of 3 hours early. By a fraction of a percent. It is an individual choice vs perceived risk.
And with more infrequent travelers flying in summer and on holidays, these warnings are periodically released for their benefit.
However, this specific email warning does not seem to reflect that anything has changed wtih TSA on a permanent basis at ATL or system-wide since DL's main website is still recommending arriving 2 hours prior. The TSA/Border story is a month old and have not seen any follow-up that it has actually happened. Last article from late May discussed sending 400 TSA workers system-wide. That is under 1% of the workforce.
Everyone has their own comfort level with airport arrival times, and a lot of that has to do with traffic uncertainly on the way to the airport and familiarity with the airport as much as TSA. Moreover, do you have a favorite planeside destination at the airport (restaurant, SkyClub) or do you just sit at the gate? All of that factors into the decision.