According to the documentation, it does this:
true if blocking ('...Sync') methods can be used.
Evidently Node Sync methods are blocking, and Crockford hates blocking operations in JS.
Note: There's a lot of stuff JSLint is picky about that may be more a matter of taste than a matter of code quality. Crockford is known to be rather opinionated about certain conventions and doesn't distinguish these in JSLint from actually harmful code. That's why there's tools like JSHint which tend to be more forgiving of these matters of taste.