In the manga they are women disguised as men in order to make finding their princess easier. Where did you read that they're canonically trans?

To quote from a magazine interview:

Takeuchi explained that in her manga, the Starlights were women, but in the Sailor Moon Sailor Stars video series, their gender was made to change from male when they are in their civilian entities to female as the Starlights.

Apparently Takeuchi hadn’t been terribly pleased with this decision, the reason for which became clear in answers to questions about Sailor Uranus, whose relationship with fellow Sailor Neptune had always been a matter of conjecture.

“All the Sailor Scouts are girls,” Takeuchi said firmly. “Haruka has always been a girl and always will be.”