Can job interview questions be confidential? Is this legally enforceable?
It is lawful, unless the nature of questions is outlawed or hinting at illegal activity. For practical purposes, though, confidentiality would be hard to enforce because Google cannot realistically prevent you from disclosing those questions. Whether or not Google can remedy any breach of that clause, that is a separate and uncertain aspect.
Google's clause is binding only if you consent to it. Signing an NDA is not the only way to render confidentiality binding. Consent can be evidenced by anything that reflects your agreement to abide by that condition. It could be an email reply, or by subsequent conduct such as attending the interview with knowledge that constraint and without objecting to it.
The fact that you attended the interview after reading that email makes their clause binding unless you objected and they nonetheless conducted the interview.