It seems that "proven jumper" is not a KPI that Cheika had for the final backrow position when he selected the group.
He included Tui in his selections and Hanigan who should be available for the second test.
Clearly there were more 6/8 options available that were better lineout options than Timu and Samu but Cheika presumably thinks those two offer more overall and he'll have a better team with the options he has than the players he's left out.
If we get punished at the lineout on the weekend with the expected backrow then that will be a fault in the selections (unless the backrow chosen adds enough elsewhere to overcome that). If the lineout functions well and the backrow goes well in other facets then the selections will have paid off.