Yeah, not sure about this one chief. I don't know the legal aspects here, but I think it makes sense to not create an open-source project that competes with your previous employer and then correspond with them letting them know you did it...
Wouldn’t this only hold if you signed a competition clause, and if so then they would have to compensate you for not being able to work on technology X for Y amount of years?
In general though, doesn't it make sense to not create a free competitor right after leaving your employer? They may have a case regarding stealing trade secrets given that they did give you access to their codebase and daily ops.