Consider a two geographically non-overlapping cell around node 4 and node 9.
As far as spatial channel reuse is concerned, the time interval [0,Time] is shared by all nodes in each cell.
From Fig.2, when node 6 or node 8 transmits node 7 can not transmit.
This means S6, S7 and S8 are nonoverlapping airtimes.
Now, nodes 6 and 8 use up 2x fraction of air time during the interval [0,Time].