Yes that makes sense as it is a 'get out' clause to protect the tester/test centre.
I agree, even after a home build I kept as much of the donor documentation as I could including a photocopy of the V5 and the scrappage certificate which included the engine number and date of registration. I then got an engineers report (as Paul said on headed paper) to confirm that the installation was 'fit-for-purpose' and that not only supported the MOT but also the Insurance and engine registration with the DVLA.
In the great scheme of things it only cost me a couple of quid but it seems this is the way to go.