There's really no need to explain it. Because...
1: It's decades ago. Storm, Jean have clearly moved way past that fictional moment.
2: It does aptly show Mastermind's reprehensible nature and power and influence. He corrupted Jean's mind and memories to the point where her truest friends were less than nothing to her. As a Black woman living in the era, "less than" meant slave.
3: It's a testimonial to the strength of their friendship...that Storm does not hold that moment against her.