It’s not the extended factory warranty - it’s the Seat-backed warranty that you can purchase when the three year warranty expires. However, whether you purchase the extended factory warranty at time of ordering a new Seat or the extended Seat warranty when the three year factory warranty expires, the same applies to servicing - no requirement for servicing by a Seat dealer, although if using a (reputable) indie, genuine Seat parts and Seat approved fluids should be used and all invoices and receipts kept to keep the warranty valid.
Clearly, less opportunity for Seat to wriggle out of honouring legitimate warranty claims if servicing is carried out by a Seat
dealership while a car’s under warranty. Also, as
@Sword has said, Seat are unlikely to entertain any form of goodwill once the car’s out of warranty if it’s been serviced outside the Seat dealer network, although since Dieselgate, VAG seem to be much less receptive to goodwill contribution requests.
Also it’s worth bearing in mind that;
At the end of the day, it’s up to the owner to decide where to get their car serviced. If an owner knows of an independent garage with a good reputation and knowledgeable staff that specialises in VAG cars, then they may get better service and better quality work at a
lower cost than they would from a Seat dealer, although there will be good and bad examples of both independent and Seat dealers.