Is Back EMF relevant in dynamos, where there is electric generation?
There is a back-emf even in dynamos that can be considered. A perfect dynamo will produce a terminal voltage related to the speed that it was spinning and this also is the same for an imperfect dynamo.

However, under load, the imperfect dynamo has an extra factor to take into consideration; sure, as the electrical load increases, the dynamo shaft is harder to keep turning at the same speed (conservation of energy) but the current taken from the dynamo also flows through a few percent of copper wire turns that are not generating a voltage - this is called leakage inductance (also applies to transformer windings) and the current will produce a back-emf in those turns that reduces the forward emf due to the perfect dynamo effect.

Maybe this is what the OP is considering?