Interesting that he's drawn a comparison between the 4-strokes, track design and injuries.
I'd basically assumed that the relatively high number of serious injuries in Australia was due to the increased depth of competition/seriousness of the competition and due to more tracks going for big jumps impress the uneducated spectators - and that the 4-stroke thing was incidental.
There was a similar spike in serious injuries in the early-mid 1990s when the country went SX crazy.