It is the classic type of thing insurance companies do. They are effectively taking a bet against something with very long odds, but with a punitive financial penalty if you are that unlucky one in a 100,000.
Your conveyancer will never be able to check for potential chancel issues on every house move they do, the piece of paper may be sat in a solicitor’s clerk’s safe dating from decades ago….
So the industry looks for a way to indemnify individual clients, make an easy profit and hardly ever have to pay out. Where it gets more questionable for me is when it is a third party (lender) insisting you have indemnity. But it doesn’t cost much (such are the odds the insurance firm are betting on), so easiest to suck it up. There are much more stressful things about moving house. Dealing with twunts in general being the main one.