For anyone else, not you TJB, I appreciate you see the flaw now, that is still not quite understanding this:
1 For instance, both teams go in a batsman heavy with a bowling sub.
Toss winner bats. Has clear advantage as they get their bowler back in the second innings. Clear advantage.
2 Both teams go in with an allrounder or batsman sub. Toss winner bowls. Swaps #11 for an allrounder/batsman. Has clear advantage.
3 A: team goes in with regular team and a allrounder/batsman sub, B: goes in a batsman heavy with a bowling sub,
If A wins the toss and bats first; No advantage at worst as B is a bowler short, if B changes for bowler in first innings, it is regular team vs regular team. Draw at worst for A - or a 6th bowler A/R benefit at best if it bats first. A will presumably bat first. (But A could bat second if its batting sub was as good as B's heavy batsman is batting heavy and get a draw.) A could bowl first and both get extra batting, the AR vs bat heavy will determine whether this is a draw or a foolish disadvantage that is stupid to attempt.
If B wins toss, it will bat first. Draw at worst or batsman over A/R advantage. If A wins toss and bats second and changes a bowler for a bat or All-rounder. Draw. No advantage. Both get the extra batting, with B possibly getting slightly better batting (not logical to do so where it happens).
So if B wins toss; it has to bat first. No advantage, or avoids the 6th bowling A/R at worst and gains the batsman over A/R at best. Draw at worst pending how good A's AR is (could still be a win). Both get batting. If B bats second, presumably it gives up its batsman to bowl first (not logical to do)
2/3 scenarios the toss winner gets an advantage (excl 6th bowling AR or batsman over AR advantage discuss above) and 1/3 scenarios it is a draw (this is being very generous given the A/R advantage it includes). The toss winner cannot lose if logical, only win, most of the time, or draw at worst.
And that is the fallacy of the super sub rule pre-coin toss. The winner of the toss had far more chances to win an advantage than draw, and a draw was their worst case scenario if they were sensible regardless of subs. Given most went with a batsman or a/r, they bowled first on toss win for a clear advantage.
Noone was going in bowler heavy with 6 frontline bowlers when 5 at most are needed and you could have an A/R on the bench? What is the point? And given the teams saw the team sheets first before deciding, toss winner drew at worst, or won an advantage, unless foolish.