Yes. However, as you can make out from the # of negative reviews some of these pro sellers have, it is evident that buyers would rather forgo the 100+ dollars they spent on the order (so that they can post a review) than get a refund, which you have already pointed out is very easy to obtain. This also highlights these buyers’ professionalism (as they believe it would be a disservice to the other buyers if they didn’t forewarn them).
Buyers who purchase from these vetted sellers EXPECT products/services that are of impeccable quality (and rightfully so). They do not deserve a product that was even partially designed/created with the help of some other non-vetted seller. If the buyer wanted something that was created (even partially) by a non-vetted seller, they wouldn’t have decided to purchase from a vetted seller.
Since all pro sellers were already “thoroughly vetted” in their areas of expertise, I think it is safe to say that they are capable of delivering products/services of impeccable quality (without needing any one else’s help). This is assuming pro-sellers do not agree to work on orders that are outside their comfort zone. So, the only way for them to receive an extremely critical or negative 1-star review would be if the product they delivered was not 100% designed/created by them. Therefore, it is safe to assume that the extremely critical and negative reviews (1-star reviews) are a result of outsourcing part/all of their work to other sellers.