Rather odd considering they were likely in their car or similar and there is no need to wear a heart monitor for the results to enter the leaderboard. Ditch the HR monitor and there would have been very little you could do. It's the one problem with Strava. If one was cunning enough (and a cheat) then one could easily just drive around segments at the desired speed. I have heard of people doing exactly this.
I mentioned before that it would be good to have an authenticity score per rider per segment. Highest score would have HR monitor, cadence sensor, speedo and power. Lowest score would be a simple speed. Strava inputs power/watts on every ride also but this is pure speculation and based on the same conditions each time.