I believe that what Jim meant is this. If you take code written in java syntax and put it in a groovy file, it will work, but if you take code written in groovy syntax and put it in a java file, it won't work. Jim was talking about the reason why some companies do not want to use groovy, since they look at it as a foreign language, and that if it doesn't do well, and there are no resources that know groovy, maintaining the code will be a hell. He also commented that this is not entirely true, since the syntax is quiet easy, and that it's going to be easy to pick up.
Yup. Also I don't think it would be desirable to try to write Groovy that is fully compilable as Java - if you do that, then you're just writing Java, and what point was there to using Groovy in the first place? Well, I suppose you can still use the various Groovy classes, and that's something. But without using Groovy syntax, you're really missing out on the simplicity and brevity that are a main part of the point of the language.