When Nancy got her Spyder, BRP was backordered on oil filters. When I asked the dealer if we should stop riding at 600 miles, they said not to worry about it. First service (after I got a filter from Firefly) was at over 900 miles. No sweat! My own RTS went into the dealer at 597 miles for warranty work. Many weeks and over 100 miles later, I got it back, and did the first service myself at 700+, with their blessing. The service intervals are not numbers set in stone, but I wouldn't stretch their 1,000 much. There are important things to do at the first service.
Your dealer is correct, in that they effectively are your warranty. If they decide not to do a valve check or other required service, the responsibility rests on them, as long as it was their decision. They are the ones you have to please, and that have to report a warranty infraction to BRP, and they appear to be satisfied. I would make a dated note of your conversation, and who you talked to, just for backup. I would also question their motives, however. Are they just too busy? Do they do this for all Spyders? If this is an indicator of an attitude problem, lack of qualified techs, a work overload, or some other difficulty, you may want to research other dealers.