I think what they mean is, they will not refund your money after 28 days, but they will repair or replace during the guarantee period, which would normally be one year.
Certainly your normal consumer rights would protect you for longer than 28 days.
From their perspective, the problem with computer parts is, they could have been mishandled (or misdiagnosed) by the customer. I'm not suggesting you have, just pointing out reality.
But my (limited) experience of Novatech, is they are a well respected company, and I would doubt you would have any serious problems with them for a genuine problem.