No, some certainly were. There's one quoted in the blog post, for example.
In a small community, and if you're not using it for lots of chit-chat but rather for something focused like scientific research, you can go years and years without reaching 10k messages. So in that circumstance, it's pretty reasonable to have chosen Slack under its old policy. Especially if one wasn't aware of Zulip. ;-)
I work with a ton of scientists (and am one myself) and I’m a member of multiple slack channels with hundreds of members that are years old and only have a 1-2k messages.
An example is a Python users group where there’s maybe a few threads a week. And it’s really useful to search for old threads.
Another is a hackathon with maybe 30 people where there were a few hundred messages over a week and then 1 message a month.
In a small community, and if you're not using it for lots of chit-chat but rather for something focused like scientific research, you can go years and years without reaching 10k messages. So in that circumstance, it's pretty reasonable to have chosen Slack under its old policy. Especially if one wasn't aware of Zulip. ;-)
(I'm one of the Zulip developers.)