Fluxer is doing the same thing, no email signups right now on its homepage.

It's no different from how lemmy/piefed function. Some instances require email, others don't. My instance, as an example, doesn't require an email to sign up, but it does require you to write a short message as to why you're interested in joining the server, and what communities are appealing to you. This weeds out 99% of bots or spammy users, and the handful that get through that are quickly banned.

Movim currently has so few users that the main server is trying to put as few barriers as possible to adoption, other servers can and do enable the Email requirement.

If it becomes more popular and bots or spam accounts become an issue, they could easily activate the email requirement, or even implement a system similar to what I described above. Instances that don't take appropriate measures to those threats as they become a problem can just be defederated as they are here. It's worked out pretty well so far.