I think it is for precisely the reason you’ve said. We have optionality - he isn’t in our best 17 (according to the coaches), can play where injuries arise over the next few rounds (with approval) and after round 11, is contracted for next year and he is going nowhere. So no need to upgrade him.
By leaving spot 30 open, and waiting, we can potentially land a bigger fish (as we’d only have to pay pro-rata for the year).
In some respects, having the top 30 loaded and locked so early was suboptimal planning. I like have this flexibility / optionality.