Anyone using a power meter is recording at 1 point per second, otherwise you're throwing away an awful lot of data.
Also the cycling specific Edge series of Garmins default (I think) to 1 point per second recording if certain accessories are detected (HR strap, cadence, power meter) unless otherwise configured.
I'd also guess that the Garmin Edge is the most likely make/line of GPSes for Strava uploads from cyclists (excluding phone apps).
A further guess it that the largest contingent of uploads to the original Digital EPO sites were originally recorded on Garmin Edge devices. It's not about cycle tourists using Etrexes.
Modification is piss easy, you just adjust the timestamps to change the speed by an uplift of say 10% or something.
As I said, coming up with a basic but vaguely convincing fake (e.g. trackpoints lat/lon/timestamp/elevation every 15 seconds) would be quite easy. Coming up with something that passes closer scrutiny is a lot harder, i.e. something that contains power, cadence and HR data at 1 second intervals.