Agree with your thought process, in that I’m not sure how literally we should take this.
If we assume Martin plays 80% TOG with a 50-50 split of that across the midfield and forward line, it still means there’s scope for Williams to split his time (say) 60% mid, 20% forward and 20% bench.
Ie Williams and Martin could theoretically “share” a mid spot, but Williams could end up with decent midfield minutes if he also plays mid while Martin is benched.
I did remove Williams following the article, but I was lineball on him anyway.