I think some people get touchy about them being lumped together if their last period of commonality (per the article) was 1400 BCE. For comparison, I believe all the Slavic languages were mutually intelligible around 1200 AD. But much more recently than this, in the last few centuries, there have been notable attempts by east slavs to absorb the Baltic language cultures and deny them.
I may be off by 100-200 years, but this is what I read. There were accents and regionalisms but they were all mutually intelligible.
It is an example I think of often, about how quickly languages can change. In the scale of 1000 years, a lot changes. Most of the diversity in Romance languages is from around that timescale too, it really started to diverge substantially around 900ad-1100ad.
Depends on your standards, too. Even today, any pair of slavic speakers should have a head start in understanding each other. Put them next to each other for a month and they should be talking, at least about basic everyday things.
