Do you have a quote about Z93 basal diversity pointing to Central Asia in the Paper? I only see the following.
Y-STR haplotype networks and diversity
We genotyped a subset of 1355 R1a samples for 10–19 Y-chromosome STR loci (Supplementary Table 3) and constructed networks for both hg R1a-Z282 and hg R1a-Z93 (Supplementary Figure 1 and Supplementary Figure 2). Although we could assign haplotypes to various haplogroups, power to identify substructure within hg R1a-M198 was limited, consistent with previous work.22, 52 Although haplotype diversity is generally very high (H>0.95) in all haplogroups (Supplementary Table 3), lower diversities occur in south Siberian paragroup R1a-Z93* (H=0.921), in Jewish R1a-M582 (H=0.844) and in Roma R1a-M780 (H=0.759), consistent with founder effects that are evident in the network patterns for these populations (Supplementary Figure 2).
Wich make them believe that Z-93 origin was likely south asia.
They literally call it the Siberian paragroup
See the joining network:
https://media.nature.com/original/nature-assets/ejhg/journal/v23/n1/extref/ejhg201450x2.pdf