For RESPA it doesn't matter if the person being paid for the referral is a settlement service provider themselves but the referral has to be for some kind of settlement service.
A home builder is not a settlement service provider, but a lender would be. If the referral involves a settlement service then it would be RESPA covered. So if the builder is paid a referral fee for referring to the lender that is a referral involving a settlement service and would be a Section 8 violation. On the other hand, if the bank was paid by the builder to refer customers to the builder for construction services then (assuming there are NO other arrangements trying to tie this activity to some thing of value or referral back to the bank for lending services) this would not create an issue because the referral does not involve a settlement service. Builders are a common source of Section 8 violations but this is because lenders go out to give them things of value for the referrals of settlement services.
To make the example more extreme, it wouldn't be a RESPA issue if the bank received a thing of value to recommend a landscaping company just because the bank makes mortgage loans.
In the OPs example, it looks like the builder is paying the realtor for a referral related to construction services which are not settlement services. If the arrangement was turned around and the realtor was paying the builder for referrals this could be covered and represent a Section 8 violation. Although, as it was pointed out, this would not involve the bank.
Opinions expressed are my own and do not reflect legal advice or the opinions of my employer.