Not exactly true in all cases, but I agree with the sentiment.DO ask to see their credentials and current insurance card. If they have insurance, they will have a card, just like you have for your auto insurance.
As one example, where DMs / Instructors are insured through a shop group policy (rather than carrying individual insurance) with Vincennes & Buckley (a primary PADI insurer), individual cards are no longer provided, as I understand it. Rather, what you get is a copy of the shop's policy, and the 'Named Insured' attachment to the policy, which lists your name among the insured staff members. It is just not a card. When I take OW students on a charter boat, for example, I provide the operator with an email to which is attached those documents.
So, I agree that getting some documentation that what you are hiring / paying for is a 'renewed and insured' dive professional (at least, in PADI-speak), there may or may not be a 'card'. Perhaps, the broader message is to be sure you are getting the credentialed dive professional you are paying for. That is true across agencies.