This is a completely normal request—and here's why.
The certificate of insurance (COI) is not legally binding on
the insurance company with respect to specific coverage details of the actual policy and its endorsements (policy amendments for special cases), and with respect to the additional insured parties. That is to say, it is a summary for convenience. Most of the time it is convenient enough—because to provide a knowingly false one as part of a bid for contracted services is simply fraud, which isn't going to be a good engineer's goal.
But what if the insurance policy contains an objectionable endorsement, like
an exclusion for professional liability? This often won't be noted on the certificate, but it would be very important to the qualifications for some kinds of contracts. The client probably just wants to make sure that there is the right insurance available—which, as a matter of fact, is so that they
don't get the urge to sue you if you screw up. The client may also be aware that a lot of people were never instructed in the details of insurance, and might be overlooking the significance of this.
Or, what if the engineer's business department just took an old copy of the certificate of insurance, and roughly edited in the name of the new client at the bottom? (Don't do this: let the insurance company generate its own certificates. It's their job, and it's no trouble for a reputable firm.) The client may be wondering how that name got there, but estimates that it is just an overzealous assistant, and that the policy itself has a term or endorsement that just says that anybody they contract with is an additional insured—and therefore the suspiciously edited certificate would be immaterial. Maybe they think that the conversation will go more smoothly if they don't voice suspicion, and instead couch it in terms of a simple administrative request.
The bottom line is, just give them the policy document. Most policies are very standard and very boring, and their disclosure won't put you at a disadvantage. And if you have proprietary information in the policy, either redact it with a black box (leaving the section heading so they know what they are missing) and note the reason for redaction (like a short comment beside it), or negotiate with the client about nondisclosure.
It's cheaper and easier for you and them to talk about it in advance, than to have a problem and argue about it later.