According to Signal's 990, it's paying multiple employees over $700k. That's above-market for corporate compensation, and it's way above market for non-profit compensation, to the point where it could be considered private inurement.
Signal is trying to compete with the richest companies in the world; including for talent. And considering Signal's origins and motivations, they're not going to lower salaries or decrease benefits because some people believe that working for a non-profit automatically means lower compensation.
This means that the pay packages are likely not based on comparable market wages, which is an actual legal requirement for highly compensated employees for U.S. charities.