Did you read the small print of the contract you signed? The extended warranty you paid for is with a company with a relationship with Harvey Norman, not with Samsung or likely even directly with Harvey Norman. HN will get a commission on each one they sell.
They are not required to supply a replacement TV while they take it away for repair. There's also no requirement on them to use Samsung parts, but the nature of current TVs means there are very few parts/modules that aren't custom units that can only be repaired / replaced by any means other than swapping a complete unit, and that means only manufacturer parts are available.
They are entitled to take it away for a 'reasonable period', trouble is, nowhere in consumer law is this reasonable period defined. What you consider reasonable and they do can be very different. If they need to send it abroad for specialist repair, or get specialist parts shipped in, that can take considerable time.
The repair should come with a warranty period covering that particular issue only, but that won't necessarily last very long.
I generally avoid any form of extended warranty, overall they offer poor value for money.