Which ones? The 10in netbooks and tablets I'm familiar with are all 1024x600.
Youtube HD has 3 resolutions:
448x336
480x360
1280x720 ("720p")
Further, 1280x720, being a standard HD resolution (720p/720i), should be able to be down-converted to another standard resolution: 480p (which is 720x480, though technically it's ED (enhanced definion), not HD). The iPad should be able to display 480p in both portrait and landscape. If they had made the iPad with 1024x600, it couldn't display 480p in portrait mode.
So, it can display the first two Youtube HD resolutions natively, and it should be able to down-convert 720p to 480p, and display those as well. All 3 should be displayable in both portrait and landscape orientations.
Plus, it should be able to display the lower 2 Youtube HD resolutions in pixel-quadrupled mode (896x672 and 960x720), but only in landscape orientation.
So, I don't see any reason why his claim that the iPad can show Youtube HD movies isn't valid.