Probably just faster than you. 5.5mph is pretty slow unless it’s a trials course! Someone more proficient than you will have ridden it on a jump bike, or an XC bike or whatever is the most suitable.
That and STRAVA is just showing you the fastest times, which will mostly be those random chances where the GPS signal ‘started’ at it’s maximum error, finished at it’s maximum error, and the sampling rate meant they gained another few seconds either end.
Quite easy to check, just click on the leader and look at the ride. It’ll be obvious if they were on a Cx bike riding nearby, or sessioning that trail.
Oh hang on ,I don’t have Strava
I don’t have typhoid either, but if we were here listing stuff we don’t have we’d be here a while, and it could get embarasing.