Buses only go so far at indicating whether a rail service is viable. Especially if, as has been pointed out, a bus to Exeter and then to Plymouth is faster than the direct bus was (although over half the bus services take between 2.25 hours and 3 hours with there being about a 2 hour gap between those being as quick as 2 hours).
If there was a service which got people from Okehampton to Plymouth in (say) 45 minutes then that would mean that it would be quicker to use the train than drive (even 55 minutes would make it comparable) whilst going by bus is at least twice as long. That would attract a lot more people to use the train than currently use the bus.
Again, although the DAL is likely to be faster than the existing route, it is not going to be by very much. Therefore you do not need many people making a significant time saving to offset that advantage. If the route through Okehampton can achieve a journey time of comparable to that of the existing route then, even on the two hour bus journey to Plymouth from Okehampton that could be a journey time saving of about 1.25 hours.
As I said previously it is unlikely to offset all the benefit of a 10 minutes* saving on the DAL. However it could make enough of a difference that the route through Okehampton has a better business case than the case for the DAL, especially given the other advantages which the route through Okehampton could have (including the possibility that it could give the DAL a run for its money).
*Although that is assuming that 10 minutes is achievable. Given that an average speed of 90mph over 8 miles is a time saving of 2 minutes over a train travelling an average speed of 60mph over the same length. Even an average speed of 120mph only saves 4 minutes. Therefore for the just over 7 miles of new track to save 10 minutes of journey time the trains would have to be timetabled to run at 120mph AND cut 6 miles off the length of existing route.
This could be achievable if the DAL went almost straight between Exeter and Newton Abbot, however this is not one of the options listed by NR in their documentation about the options being looked at. As such it is likely that any journey time savings from the schemes being looked at by NR would be less than this.