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.
I was in Crimea for about 2 weeks (in 2012) they split me Russian there. I couldn't understand a word they said. And I didn't learn to understand than for 2 weeks of travel there.
I could understand some words from Ukrainian (I traveled by train from Lviv).
Another example is Croatian, I've been there on vacation and renting a room. I couldn't understand a word they said and didn't learn any.
TlI can understand some Czech (because this is the closest language together with Slovakian to Polish) but that's it.
I wouldn't mix Slavs from different groups together. They evolved separately and are as close as English and German.
I doubt West and East Slavic were. But inside those geographic groups they probably were (Czech and Polish AFAIR were around that time).