^ that's just a snippet, though. Who knows what went down in their relationship. There's nothing else to conclude this was all in her head, self-sabotage or her subjective perception as you suggest. Identity theft is very common, especially with male-female dynamics in creative fields and they're talking about the time around his debut. That Margiela didn't sink means nothing, a good investor and PR, people genuinely believing your work is one-of-a-kind and borrowing from new characters every season to maintain momentum has been the recipe to reach stardom in fashion for years, especially with its tendency to overlook context and revise history. 
We don't even know if fashion was her long-term passion or if she enjoyed being in the industry, she was pretty reclusive and became a teacher just like Walter. Dries and Ann were closer but they also created more commercial clothes, I wouldn't depict their success as a symbol of failure for the other 4.