You may be right, as the check for reviews, does not show on the product review documentation, and I just checked some of my clients who have reviews, and are not listed as well.
So I don’t know how to check, because I have clients with trust pilot, and these reviews are not imported to Google Merchant Center, and these products are both product and seller.
But I do know, that reviews from yotpo are not visible in Google Merchant Center, as I have a client with lots of reviews, and it never shows, but do show on the product ads and free listings.