You've got to imagine that, as you say, having OO CADs helps as a decent starting point for the TT CADs, the corollary is that having TT CADs would mean that they have a decent starting point for the O CADs and so a step up on creating a new/better OO model.
Admittedly were it me I'd not bother with the Class 66 as there are one or two other reasonably-nice OO models already in existence with other suppliers... But you'd think they'd contemplate saving development time by ensuring that any effort spent on One Scale could be passed over to the Other Scale as a "here's one we prepared earlier".
You then think how important Hornby think the Class 66 is - they are willing to create a TT-only-use CAD for it.