KfW70: Heating - where are the pellet stoves?

And that then means, conversely, that a KfW70 new building at H'T would have to correspond to a KfW70 old building?

Basically yes, provided that over the years "KfW 70" really means the same standard. Today, for example, there is no KfW 70 anymore, the first KfW level is KfW 55. The former KfW 70 is today’s house according to the Energy Saving Ordinance.
Have a look at the Wikipedia article on energy standards. There is a table there that might clarify that.

Does that make sense?

Yes.

It might also be important to know that not only requirements according to the Energy Saving Ordinance have to be met, but also those according to the Renewable Energy Heat Act. This includes requirements such as the obligation to integrate renewable energy sources into the heating system.