TBH in that match Fischer started his shenanigans about demanding all sorts of things which most likely unbalanced Spassky - who was known to be somewhat lazy - so with all the political pressure and mind games he likely decided that he doesn't care enough.
The match was actually not that top-level chess as it is presented in chess mythology. Reshevsky [1] said the following things about it:
"True, there were several excellent games, but the match as a whole was disappointing. It was marked by blunders by both players. The blunders committed by Spassky were incredible. In two games, for example, Spassky overlooked a one-move combination. In the first, he was compelled to resign immediately, and, in the other, he threw away all chances for a win. Fischer was also not in his best form. He made errors in a number of games."
... and he is also very critical of how Spassky was prepareding for the match. So a well-prepared Karpov would likely have been able to dethrone Bobby, and I think Fischer knew it, and it played a big part in why he had forfeited the defence.
[1]: http://billwall.phpwebhosting.com/collections/The%20Fischer-...
granted it becomes more clear that fischer dominated in the quarterfinals forward, but it still is worht pointing out that his opponents weren't nearly as strong as nepo's opponents, relatively speaking
I imagine nepo's score would've been MUCH stronger if he was playing 100-200 ELO points weaker opponents all candidates. Instead nepo had 3 opponents with higher ELO and his weakest opponents ELO was only 19 points lower than his own...
in summary, I think you're correct and I was wrong, but I'm not so sure it's quite so clear-cut when you factor in other variables
What about Fischer? The format was a little different then, but here was his route to the Championship.
1. Won the Interzonal tournament 18.5/23 (+15-1=7). The top 8 from the Interzonal then played a knockout series of matches using a bracket system.
2. In his quarter-final match of that he beat Taimanov 6-0.
3. In his semi-final match against Larsen he also won 6-0.
4. In his final match against former world champion Tigran Petrosian, he won 6.5-2.5 (+5-1=3).
In the 21 games total that his candidates matches lasted, he won 17 games, lost 1, and drew 3. If we include the Interzonal it is 32 wins, 2 loses, 10 draws. In the combined Interzonal and knockout matches he had a streak of 20 consecutive wins against the world's top players.
And then in the Championship against Spassky he won 12.5-8.5 (+7-3=11) (and one of those loses was a forfeit when he skipped the game over some complaint about the playing conditions).
Nepo's performance in this Candidates was pretty dominating, but doesn't come anywhere near Fischer's level of dominance.