Does pay transparency necessarily mean that someone can't get a raise? In this theoretical scenario, Jane would know exactly how much she's being paid in comparison to the other engineers. If she's performing better than them while getting paid the same, she would know that she's worth more.

The scenario you've described seems to imply that the company doesn't already know everyone's salaries. Pay transparency isn't what kept the company from giving her a raise; in fact, without pay transparency they could have told her that she was making more than her coworkers.

Point being that companies can already refuse to give a raise by saying some boilerplate like "you're being competitively compensated for an employee of your seniority, and unfortunately we can't give you a raise."