tl;dr: this is not an "age reduction breakthrough", it is both A) a confirmation of what we already knew, which is that parabiosis makes old mice healthier, and B) a finding that virtually proves the mAge clock has nothing useful to say about lifespan
Parabiosis does not substantially extend lifespan (possibly not at all), based on the experiments done so far. Even if it did, it definitely does not double it, which is what would be predicted by naively looking at the halved DNA mAge on the clock.
This paper is very likely intended as another validation of Horvath's mAge clock. The clock shows that there is a strong association between epigenetic state and both chronological age and morbidity risk. Exactly why that is, and what it means, is highly controversial.
Mainstream aging field does not really care about lifespan much anymore. Increasing amount of focus is on so-called "healthspan" - approximately the duration of healthy life, or incidence of morbidity. You would think that you could not affect one without the other, but in fact you can, to a frustrating and surprising degree.
Therefore when you read about "rejuvenation", we are increasingly finding that you can find treatments which improve a broad spectrum of unrelated health markers, that these treatments will affect epigenetic loci predictive of age, and all of this without any substantial effect on lifespan. (From a public policy perspective, this is considered highly desirable, as for public expenditures it would be ideal if people lived perfectly healthy until the moment they keel over, even if we cannot extend the time until they do)
One possibility to keep in mind is that rodents die almost entirely of cancer. So it is possible that some treatment which improves "everything except cancer" would not show any lifespan effect in rodents. This would be one way of reconciling this paper with Horvath's earlier finding that the clock predicts all-cause mortality in humans.