To set-up front indexing, ideally you need a cable adjuster. On a MTB this is part of the shifter which we don't think about and take for granted. STIs typically do / did not have an integrated cable adjuster at the lever end.
The only standard solution back then was to place the cable adjusters on the now "defunct" gear lever bosses (although the rear derailleur also had a cable adjuster, the front derailleurs never did). At least you could adjust indexing on the fly without a dismount.
I'm pretty sure (read 99.999999%) that no solution was available back then for a cable adjuster running along the top-tube and down the seat-tube. Neither in-line nor specific braze-on cable adjusters were available back then. Alternatively, he could have put a gear lever boss on the seat-tube, but that probably would have tipped the weirdness balance a bit too much
This is how Moots solved it for top routing in 2012 .... much later.