You could argue both ways, and it would be cheesy to take it, but as a opposing view or something else to stir the pot so to speak...
When are models in Cover? (pg.21) - When any part of the target model's body (as defined on page 16) is obscured from the point of view of the firer, the target model is in cover.
Firer = Exploding TruKk
Target = Flying Orcs
Like I said RAW you could argue it either way, but logically the Orc is standing on or in the Trukk so there would be nothing obscured between the Firer and the Target. Now if any Orcs are in the area take the Str3 explosion from the Trukk I would say yes those Orcs would get it no question.