My gut feeling is that improving the worker code was a slight judgement error. This is because the "Sid Meier Fun" test was not heeded with this improvement. Yes it is more fun from a purist point of view to see workers behaving more sensibly, but it is also more fun to see them behaving a bit less sensibly, and being able to nab em good every now and then. Thus it only produces a neutral (+1-1=0) result and was not a genuine bug in the first place (it was only an imperfection).
I must admit that I find it hard to believe someone would put forth an argument that an improvement to the game AI is a bad thing... As to whether it's a bug, barring some extremely stringent requirements for the definition, I don't think there's any question there. AI workers are designed to "run away" when they sense danger, the fact they couldn't properly assess the danger from units with certain types of promotions is clearly an oversight.
Bh