If one must know both in order to be an agent in the real world then its utility is somewhat limited.
Spider does look like it would actually be a really cool first language to learn!
Yep, this is important to avoid the "Groovy" problem where Groovy is valid Java, except for lots of little cases where it isn't, e.g. == behaves differently, default member access behaves differently, etc etc. Since Java 8 lambdas, Groovy is now wildly different in its syntax.
The point being made is that if Spider wants to take off, it had best not require an intimate knowledge of javascript - and we can look to groovy to see what happens otherwise.