The brilliance of Robert Mueller's strategy with Flynn. [View all]
Limiting the plea agreement to a single count of lying means that Ty Cobb can assure DT that all Flynn is pleading guilty to is the same lying that he was supposedly fired for -- nothing new.
This might reduce the chance that DT panics and fires Mueller -- at least for the time being.
OTOH, the plea agreement contains language that leaves open the possibility that Flynn might be indicted for other crimes that were not included in this agreement.
So the agreement applies only to the particular crime in the Statement of Offense -- but in exchange it requires Flynn to give unlimited assistance to Mueller in his investigation, in any Federal or State or local prosecutions.
In other words, Mueller has gotten all the cooperation he needs from Flynn while revealing only a single one of his legal cards to DT -- and still threatening Flynn with other cards, if Flynn isn't helpful enough.
ON EDIT: Also, by reserving additional charges, Mueller has avoided the chance of double jeopardy. If DT pardons Flynn, there remain other unpressed charges against Flynn that could be transferred to a State Attorney General for prosecution.