The HM was patterned using 248nm DUV lithography and a SADP process, which resulted in relatively large line-edge roughness (LER). What's important is that the TII-defined edge is self-aligned to the HM edge on the right in this plan-view scanning electron micrograph. It is clear that the unimplanted (unetched c-Si) region (labeled #2) is very uniform in width. Moreover, as shown in Fig. 12 and Fig. 13 in the T-ED paper, TII-defined edges have lower LER than the HM edges.
The masking layer is very thin (5 nanometers) so that a very low ion acceleration energy can be used; hence the projected range (Rp) of penetration into the hardmask is very small. This can be seen from Fig. 11 of the T-ED paper. In short, the overlying HM features do not need to be 60 nm wide.
Photoresist indeed can be used as the "HM" material for TII double patterning (cf. figure 2 in this article), because implantation is not a high-temperature process. Experimental results can be found in the PhD thesis at https://people.eecs.berkeley.edu/~tking/theses/pzheng.pdf. For SADP, on the other hand, it would be difficult to use photoresist as the core or mandrel layer, because it would have to withstand the spacer deposition process temperature (greater than 150°C).