Ok, well here's how I could see the argument going (not that I'm committed to it so, as I say, probably not worth having):
p. 91 simply refers to the number of models in the unit, not the number of models in the unit for purposes of transport capacity. True, it then compares this number to the transport capacity of the transport, but that doesn't mean that the number originally referred to is the number-for-transport-purposes.
If someone points to the unit of six Grots and says 'how many models in that unit?' the natural reply is 'six' not 'it depends for what purpose you ask'. Six (the number of models) is not more than ten (transport capacity), so - the argument goes - they can take the Raider.
Thus, being Bulky means each Grot takes up the space of two models if they get in the Raider, but it doesn't mean that they count as a unit of 12 for purposes of whether they can take a dedicated transport in the first place.
As I said at the outset, this probably isn't the intention; I never questioned the RAI. But if the intention was that units can't take a vehicle that they can't fit in, then an alternative wording could have made this clearer or more explicit. RAW do seem to leave scope for someone to argue this to me.