How to interpret a postdoc rejection letter?
There isn’t anything to interpret. The only conclusion you can draw is that there was another candidate whom the professor thought would be a better fit for the position, for an unspecified reason.

Our minds have a psychological tendency to want to fill in a lack of information with speculation and beliefs. The added information you have in this case is literally zero, so you must resist the urge to “interpret”; it is futile and serves no useful purpose.

Good luck with the job search!