Footnote #40 to TIL [tied to 226.18(c)] states:
40 Good faith estimates of settlement costs provided for transactions subject to the Real Estate Settlement Procedures Act (12 U.S.C. 2601 et seq.) may be substituted for the disclosures required by paragraph (c) of this section.
The Commentary to this section states:
4. RESPA transactions. The Real Estate Settlement Procedures Act (RESPA) requires creditors to provide a good faith estimate of closing costs and a settlement statement listing the amounts paid by the consumer. Transactions subject to RESPA are exempt from the requirements of §226.18(c) if the creditor complies with RESPA's requirements for a good faith estimate and settlement statement. The itemization of the amount financed need not be given, even though the content and timing of the good faith estimate and settlement statement under RESPA differ from the requirements of §§226.18(c) and 226.19(a)(2). If a creditor chooses to substitute RESPA's settlement statement for the itemization when redisclosure is required under §226.19(a)(2), the statement must be delivered to the consumer at or prior to consummation. The disclosures required by §§226.18(c) and 226.19(a)(2) may appear on the same page or on the same document as the good faith estimate or the settlement statement, so long as the requirements of §226.17(a) are met.
I don't see any where that indicates the loan has to be a purchase. The requirement is that the loan is subject to RESPA. I often see banks use this short cut for HE and HI loans.