Based on the graphs below you might say that this decline in answers seems to be nicely filled up by the data stackexchange. But I do not think that this is like a Gondwanaland falling apart into an Africa and America that fit so nicely together.
The answer rate curve of 'stats' has a stall/decline like most other (older) stackexchange sites in the science corner (math, mathoverflow, physics, chemistry, biology). So there is already explanation like many other sites also having lower activity.
The answer rate curve of 'data' has a nice stable/rise with an extreme sudden doubling (during only a few months) in answers and questions from 800 per quarter to 1600 per quarter around new-year.
These numbers are only small and would not severely harm the statistics site. That is assuming that there is even a strong causal relation. But the statistics site did not suddenly loose 800 answers per quarter (although for the statistics site at the peak, the first quarter of 2017, there were 5410 answers and now a year ago first quarter in 2018 there were only 5068 answers, this decline does not match the 800 increase at data site which came in a more abrupt way and a slightly different moment, a few months later).
Certainly there might be some causal influence but it seems unlikely to me that this entire piece of the data site can be added to the statistics site and I imagine that the effects are only small.
I would say that we can better use the sites for what they are good at individually (I see a difference in the questions, along the lines of the technology science distinction how the sites are currently classified by the stackexchange overview, and this enhances the experience). I suggest to approach them as reinforcing each other and with this as starting point the sites can grow alongside of each-other, together, more strongly rather than having a slightly smaller platform when they would be merged together.