In "Miraculous: Tales of Ladybug & Cat Noir," Hawk Moth (later known as Monarch) seeks to akumatize people who have strong negative emotions, such as anger or despair, to turn them into supervillains to obtain their Miraculous and ultimately achieve his goals. In the case of Sublimation, who is a character that feels rejected and frustrated because of her inability to connect with Ladybug and Cat Noir, her emotional struggle makes her a prime target for Hawk Moth.
Hawk Moth likely chose Sublimation specifically because she felt a deep sense of isolation and powerlessness, which made her vulnerable to akumatization. While it's true that Sublimation understood that Ladybug and Cat Noir were keeping their identities secret, her feelings of inadequacy and frustration clouded her judgment, allowing Hawk Moth to manipulate her emotions and turn them against her.
The plot twist here is the character's inner conflict and how it makes them susceptible to manipulation. Even if Sublimation could logically assess that Ladybug and Cat Noir couldn't reveal their identities, her emotional state made it easier for Hawk Moth to exploit her despair and akumatize her. This reinforces the series' themes of emotions and how they can be both strengths and vulnerabilities.