They'd be admissible, but mostly just in an advisory sense. Policy documents like this don't, themselves, have the force of law, they just suggest the agency's preferred interpretation of existing statutes or regulations. That does matter -- if said existing statute or regulation is ambiguous, and the way the agency interprets it is a plausible one, courts will often defer to the agency's judgment as to how to resolve the ambiguity (these are called Chevron and Auer deference, respectively for statutes and regulations). But a court could also find that the existing law isn't ambiguous, or the agency's interpretation isn't plausible, or that the policy position constitutes new law that requires APA rulemaking, so they're not bound by a document like this necessarily.