> Does this imply that DoD will be updated after every Sprint Retrospective?
A Sprint Retrospective is an inspect-and-adapt opportunity to improve the team's implementation of Scrum, including their DoD.
If there is no evidence of a problem then there is no need for adaptation. This is true of any event. The Guide says that adaptation of the DoD is performed "as appropriate".
There is an implication that if no problem appears to be in evidence, then this could be because the team have not inspected thoroughly enough. Absence of evidence is not evidence of absence. In theory at least, there should always be an opportunity to improve product quality and hence the DoD. The challenge lies in finding those opportunities within the time box available.