I think its clearly false that motivation is an inherent trait. That would imply that demotivation is also inherent, which I think is even more obviously wrong.
There are all sorts of things like depression, cynicism, past experiences, etc. that can lead to someone have a lower baseline of motivation. It's also highly contextual, which I think is what you're saying and I 100% agree with. Some people thrive in role A and would want to bang their head against a wall for 40 hours in role B. Others vice versa, others would be meh in either, etc.