The reality is exactly what the owner told the poster and what Jesse said -- generally speaking, insurance companies will require a check with an instructor unless you have some number of PIC hours in the Grumman type you'll be insuring with them.
And it's funny that insurance companies don't care about the checkout CFI's time in type, because if that same CFI doing the checkout was buying the airplane for himself and had no time in type, he'd be required to find another CFI (with possibly no time in type) to do the "checkout". So he'd be fit to checkout someone else in the airplane, just not himself. So if you're using your own airplane to get checkout out in, make sure your CFI has insurance. Lots of freelance CFIs don't have insurance. Or add them to your insurance policy for the purposes of the checkout process. Hint, if the insurance company charges extra for this, you probably don't want to use that CFI.