My tuppence worth based on nothing more than personal experience.
Do lots of weightlifting. If I perform plenty of dumbbell curls I get very good at doing dumbbells curls, but this doesn’t necessarily mean I’ll be particularly great at another arm exercise I haven’t spent much time doing.
If I spend most of the winter riding outside, come the Spring I’m better at riding outside than if I’d spent the whole winter riding a bike indoors. However, if I then rode on the turbo, I’d struggle to train at the threshold power from the previous winter’s indoor training.
Conversely, if I spent the whole winter on the turbo, my threshold power would increase but, personally, this wouldn’t be replicated out on the road. At least not immediately.
There’s a big element of specificality involved in my opinion. But training is training, it’s just that there’s a period of adjustment or ‘acclimatisation’ involved when switching from one to the other.